Define scroll lock key permanently

Hi, I have to switch between two computers (KVM) and to so I must key scroll lock key twice.

The problem is that scroll key ii dead in my keyboard (US).
Typing xmodmap -e ‘add mod3 = Scroll_Lock’ solves the problem but it is not permanent (after reboot).

My question: where can I define the above command so it is available after a reboot?

Thanks in advance