I have exactly the same "problem"....
On validate on this field, Add Server Side code embed
! Servers SIDE
lok:sif_art=format(p_web.gsv('MRS:SIF_ART'),@N06)
ART:SIF_ART = lok:sif_art
ACCESS:ARTIKLI.TRYFETCH(ART:Art)
IF ERRORCODE()
p_web.ssv('MRS:SIF_ART','')
p_web.setvalue('SelectField',clip(loc:formname) & '.MRS:SIF_ART') ! on errorcode() user enter any value that not exist in table, or leave it empty...
p_web.ssv('NotExist',1)
ELSE
p_web.ssv('MRS:NAZIV_ART',ART:NAZIV)
p_web.ssv('lok:naziv_artikla',ART:NAZIV)
p_web.ssv('MRS:SIF_ART',ART:SIF_ART)
lok:naziv_artikla = art:naziv
loc:comment = p_web.Translate(lok:naziv_artikla)
END