Default value for drop down

Hi all,

I have three values in a drop down (say) 01, 02, 03.

This drop down is created in a subscreen for a standard transaction XK01.

I have to set the default value as 01/ 02 by checking the other value present in other screen.

I have done few validations for the TCode XK01 in the userexit EXIT_SAPMF02K _001.

In order to set the default value where must the coding be done(userexit/ in PAI where i have created the screen)?

An example for setting default value in a drop down would be helpful.

Regards,

Rajkamal.