Default values based input field on module pool screen

Hi,

I have two fields on my module pool dialog box screen. When I enter the value of the field 1, the description of the field should auto-populate into the value of the field 2. I cannot use PAI user-command as the screen itself is a dialog box and will consists of only two commands 'ENTER' and 'EXIT'. The field 1 is an I/O field and cannot be assigned a fcode as well.

if i enter <field 1 > = '123' then <field 2> should automatically get the value 'ABC' without having to click on any user command.

How do I achieve this requirement in module pool dialog box programming?