ABAP : How to move cursor to next field?

In screen painter, I have two fields er_id and er_name.In er_id I have validated
for duplication record.Once user enters er_id,I checked for its existence in its table
if exists warning message is displayed and if not exists cursor should go to next field i.e er_name.How to acheive this?My code is
select * from t1 where er_id = er_id
if sy-dbcnt <> o
message 'er id exists'
else
set cursor field 'er_name'.
endif
But this is not working,i placed above code in module m1 input',some said that set cursor should be in PBO.So I assigned boolean value to one variable and checked in PBO,even then its not working.
else
status = 'yes'.
endif
PBO:
if status = 'yes'.
set cursor field 'er_name'.
endif.

Help me.Thanks in advance.