Hello Sander.

How can I create a textbox that selects the value when I execute a mouseclick on it.

You need code like this in the GotFocus() of the textbox:

WITH This
  IF .SelectOnEntry
    TextBox::GotFocus()
    .SelStart = 0
    .SelLength = LEN( .Text )
    NODEFAULT
  ENDIF
ENDWITH

You need to execute the VFP base class behavior (Textbox::GotFocus()) before you set the SelStart and SelLength and supress it afterward (NODEFAULT) because the default behavior of the native GotFocus() behavior is to reset SelStart and SelLength.