Besides having a modal window pop up to grab the value you could control it from a textbox. The Focus could be set to the box where they enter the supplier code when they first enter the form,, and your code is in the lostfocus, something like:

** Put your code to verify the supplier number if needed here
** for example if its ok then llpass=.t.

IF llpass
this.enabled=.f. && I've never been a big fan of making things vanish
thisform.txtNexttextbox.setfocus && If it's ok let them continue on in the form
ELSE
Wait Window "Invalid Supplier code - please re-enter!"
NODEFAULT && So they can't leave this field til they fill it in first
ENDIF

** You may want to make allowances for someone to be able to click a Close or **cancel button and not have to fill in a code if they are simply leaving...

--
JErry