Add I/O field on custom screen using SE51

Hi,

I have custom screen and I want to add one text field and one input/output field on it. Is it enough to drag and drop in layout view, or I have to do some coding in logic flow view?

PROCESS BEFORE OUTPUT.

MODULE PBO_0100.

*

PROCESS AFTER INPUT.

MODULE USER_COMMAND_0100.

Also, what is the difference in coding if I want this input/putput to be declared as custom filed, and

as field from original sap table? where should I declare it, in PBO?

If I use this field as original sap field (for example KNA1-TELF1 CHAR 16), is it possible to put restriction on it, so that first two character be digit, for example 77 ??

Thanks a lot,

Nihad