Create a Button in Purchase Invoice Screen

Dear Experts,
I want to do some action in purchase invoice screen ,
for that i want a button in this screen, please let me know the solution.
Thanks and Best Regards,
Dhanapal Amulraj,

 

Thanks a lot.. Its working fine

Thanks,
Dhanapal Amulraj,​

 

i have customized my invoice but i could not get disc.% in my invoice printing and it is very urgent. pls any body help me and tell me code for the same