[SOLVED] Autofill current field from another table

Edit: I have decided my question below is not complete enough to receive a functioning reply, I therefore will mark it solved but wish I could delete it. Kindest regards,

Hoping a forum member will help me with a script I am in need of, I am trying to find a work-a-round for autofill that's not working for me.
I did spend several hours searching thru forum and google for answer to below question, did not discover, so I am here at forum.

I can not get autofill to fill from lookup table in previous field: Vendor, therefore, I need to autofill from events. TransMemo is not readonly... and both table fields have same table/field settings.

Two tables: 

tblRegister and field = TransMemo ... would like to autofill this field: TransMemo with data from tblVendor, field name: TransMemo.

I would prefer to make this happen within tblRegister/Add page/Before record added event.

I just pulled below code from the air as example for code that does not work... I need a simple script that does work.

$values["TransMemo"] = tblVendor($values["TransMemo"]);