Check Screen Fields in ME21n

Hi,

I have to put a check on Document type and plant combination in ME21n.

After i enter values in Doc Type and Plant and i hit enter it should perform a check.

Check Logic:

I have made a Z table and if the Doc type n plant combination does not exist in that Z table then it should throw an error.

Now my query is,

which exit or badi should i use? i cant find anything related to it.

Thanx in advance

Regards,

Shraddha