Make a voucher mandatory?

[User1234567891]

Is it possible to make the entry of a voucher code mandatory in the chekout process?

 

For a client I need to add a field in the checkout process where registered users have to enter a cost code.

This field has to be mandatory.

 

I thought it is handy if i can just use the voucher.

 

perhaps there is another way?

[User1234567891]

No any has a clue?

 

Or maybe a tip to a great module which adds and extra mandatory field in the checkout process?