Userform and Selecting Cell values

Rather than clicking, why not load the values in those textboxes in the UserForm_Initialize() event? Something like

Private Sub UserForm_Initialize()
    TextBox1.Value = Sheets("Sheet1").Range("K6").Value2
End Sub

If you really want to select a cell (after the userform is shown) and then populate the textboxes, then yes that is also possible. Here is an example

For demonstration purpose, I am going to populate 1 textbox. Let's say our userform looks like this. Note, I added a CommandButton1 next to the textbox. I changed the caption to .... We will use Application.InputBox with Type:=8 so that user can select a range.

Next paste this code in the userform code area

Option Explicit

Private Sub CommandButton1_Click()
    Dim rng As Range
    
    On Error Resume Next
    '~~> Prompt user to select range
    Set rng = Application.InputBox(Prompt:="Select the range", Type:=8)
    On Error GoTo 0
    
    '~~> Check if the user selected a range
    If Not rng Is Nothing Then
        '~~> Check if the range is single cell or not
        If rng.Columns.Count > 1 Or rng.Rows.Count > 1 Then
            MsgBox "Select single cell"
            Exit Sub
        End If
        TextBox1.Text = rng.Value2
    End If
End Sub

Demonstration