How to change the initial directory in a browse dialog

So, i have a couple of fields that when a user clicks on them, they can set an image or pdf for the field.

how it works now is this script
var s = this.getField("Platzierungsfeld 2")
s.buttonImportIcon();

it works fine, howeever, I would like to set the initial directory of the browsing dialog to specifc folder.
is there a way to do that?