HTML button to NOT submit form

I have a form. Outside that form, I have a button. A simple button, like this:

<button>My Button</button>

Nevertheless, when I click it, it submits the form. Here's the code:

<form id="myform">
    <input />
</form>
<button>My Button</button>

All this button should do is some JavaScript. But even when it looks just like in the code above, it submits the form. When I change the tag button to span, it works perfectly. But unfortunately, it needs to be a button. Is there any way to block that button from submitting the form? Like e. g.

<button onclick="document.getElementById('myform').doNotSubmit();">My Button</button>