How to add input check on custom fields in /SAPAPO/MAT1

Hi,

I've added some custom fields in transaction /SAPAPO/MAT1, they're in /SAPAPO/MATKEY-ATT01-05.

Now I want to add an input check on those fields because they can only have certain values (i.e. FIELD /SAPAPO/MATIO-ATT03 MODULE xxx).

How can I do this?

Tia,

Filip.