Did you figure this out? I'm looking to do something similar.
I did find some code that got me started. Here it is:
'autopopulate a field based on entry in another field
Sub AutoPop
Dim objDBF, strCode, strTest, i , strObs, pageGen
'pageGen = Layers("SignsTest3").Forms("EDITFORM").Pages("General") 'SignsTest3 is the layer
strObs= Layers("SignsTest3").Forms("EDITFORM").Pages("General").Controls("domDescription").value 'get value user selected in domDescription field
Set objDBF = Application.CreateAppObject("recordset")
objDBF.Open "C:\Users\mbishopp\Documents\SignsTestData1\dbase\Example.dbf", 1 'open the external DBF table <--this should probably be placed somewhere else, like with the apl or axf, but I was just experimenting at the time.
objDBF.MoveFirst
For i=0 to objDBF.RecordCount-1 'loop through DBF list until end of list
If strObs=objDBF.Fields("Descriptio").Value then 'compare selected obs value form with 1st CODE value from table
strTest = objDBF.Fields("SignType").Value 'if form value = CODE value then store associated TEST value in variable
MsgBox(strTest)
Layers("SignsTest3").Forms("EDITFORM").Pages("General").Controls("tbSignType").value = strTest 'Populate the tbSignType control with var strTest
Layers("SignsTest3").Forms("EDITFORM").Pages("General").Controls("tbDescription").value = strObs 'Populate the tbDescription control with var strObs
'ThisEvent.Object.value=strTest 'enter stored variable into text box on form <--didn't use this, legacy from other user that I stole this from :)
Else
End If
objDBF.MoveNext 'goto next record in DBF table
Next
objDBF.Close 'close DBF table
Set objDBF = Nothing
End Sub
Hopefully that helps!