MockMvc - Pass an object from the mock to the controller

I am doing unit tests of a web application that I am developing and I have encountered a problem when testing some of the functions of the controller.

Basically I am creating a mockMvc and I want to pass it an object that I previously created. The code is this:

        Connection connection1 = new Connection();
        connection1.setStatus(Status.IN);
        connection1.setConnectionId("countingCamera1Conn");
        connection1.setPath("urlPath");
        connection1.setUsername("userName");
        when(connectionRepoMock.existsById(anyString())).thenReturn(true);

        //then

        mockMvc.perform(post("/model/connection")
                .content(asJsonString(connection1))
                .contentType(MediaType.APPLICATION_JSON)
                .accept(MediaType.APPLICATION_JSON))
                .andExpect(status().isCreated())
        ;

The controller receives two objects per parameter. This controller is the one that uses the view to create a new record of the indicated object.

    @PostMapping("model/connection")
    public String addConnection(Connection connection, Model model) {
        checkRole(model);
        if(!checkElement(connection,model))
            return "error";
        if(controllerRepo.existsById(connection.getConnectionId())) {
            model.addAttribute("errorMsg", "The Id already exists, please try another one");
            return "error";
        }
        controllerRepo.save(connection);
        return "redirect:/model/connection";
    }

I have verified that if in the addConnection() method I put the @RequestBody tag, the test works for me but it stops working from the Web. How can I simulate the object the controller receives with the mockMvc?

From the view, I'm using "thymeleaf" to generate new records from HTML form. The source is like this:

        <div class="form-group row px-md-4">
            <h1>Connection</h1>
            <div class="col-sm-1 py-sm-1">
                <button class="btn btn-primary" type="button" data-toggle="modal"
                    data-target="#addConnection" value="Add a connection" rel="tooltip"
                    title="Add a new connection">
                    <i class="fa fa-plus"></i>
                </button>
            </div>
            <!-- Modal -->
            <div class="modal fade" id="addConnection" tabindex="-1"
                role="dialog" aria-labelledby="#addConnectionTitle"
                aria-hidden="true">
                <div class="modal-dialog modal-dialog-centered" role="document">
                    <div class="modal-content">
                        <div class="modal-header">
                            <h5 class="modal-title" id="addConnectionTitle">Add a
                                connection</h5>
                            <button type="button" class="close" data-dismiss="modal"
                                aria-label="Close">
                                <span aria-hidden="true">&times;</span>
                            </button>
                        </div>
                        <div class="modal-body">
                            <form th:action="@{/model/connection}" th:object="${connection}"
                                method="post">
                                <div class="form-group">
                                    <label for="id_input" class="col-form-label"><b>ID:
                                            *</b></label> <input type="text" class="form-control" id="id_input"
                                        th:field="*{connectionId}" placeholder="Enter ID" required>
                                </div>
                                <div class="form-group">
                                    <label for="path_input" class="col-form-label"><b>Path:
                                            *</b></label> <input type="text" class="form-control" id="path_input"
                                        th:field="*{path}" placeholder="Enter Path" required>
                                </div>
                                <div class="form-group">
                                    <label for="username_input" class="col-form-label"><b>User
                                            name: *</b></label> <input type="text" class="form-control"
                                        id="username_input" th:field="*{username}"
                                        placeholder="Enter User">
                                </div>
                                <div class="form-group">
                                    <label for="pwd_input" class="col-form-label"><b>Password:
                                            *</b></label> <input type="password" class="form-control" id="pwd_input"
                                        th:field="*{pwd}" placeholder="Enter Password">
                                </div>
                                <div class="form-group">
                                    <label for="status_input" class="col-form-label"><b>Status:
                                            *</b></label> <select class="form-control" id="status_input"
                                        th:field="*{status}">
                                        <option
                                            th:each="status : ${T(com.ascspain.iothub.model.Status).values()}"
                                            th:value="${status}" th:text="${status}"></option>
                                    </select>
                                </div>
                                <label class="text-muted px-sm-1"><small>Check
                                        that all credentials with * are filled</small></label> <br>
                                <div class="modal-footer">
                                    <button type="button" class="btn btn-secondary"
                                        data-dismiss="modal">Close</button>
                                    <button type="submit" class="btn btn-primary">Save
                                        changes</button>
                                </div>

                            </form>
                        </div>
                    </div>
                </div>
            </div>
            <!-- End of Modal -->
        </div>

Thank you very much in advance.