Help with ignition-on fuse or wire tap

I ordered and installed an auxiliary switch box, got the box mounted and wire ran through to driver side. The iddue I’m having now is connecting the fuse to power. I quickly realised the under-hood fuse box is a pretty sealed box and won’t realistically get a tap in there, and the box is a long way from the interior fuse box. Has anyone had to splice onto an exiting ignition-on wire (possibly in the headlight) and had success?

Thanks for any help