In the "Calisanlar" catalog, for the attribute "ID" you have to change the type of attribute to UUID (Screenshot 1).
In order to show the full name of the user on the elements of the form, we first add an attribute of the form which is also called "ID", but this time it's data type is string 25 characters. Then drag and drop it to the elements of the form window to the left. (Screenshot 2)
Lastly, on the module of the form you should commnet the line "thisform.object.ID=GetIDUser(SelectedValue);" and add the correct line "thisform.ID = GetIDUser(SelectedValue);"(Screenshot 3)
I have uploaded the .dt with working functional too.