Focus on Form (C#)

In my program, I click a button to go to a different form, and on the form it always focuses on a (read only) textbox (highlights its content).

How do I get it to focus on the form instead?

I've tried using .Focus() and ActiveControl.

Active Control worked, but I got an error that said "control not found", but then when I clicked "continue", it loaded up the form just fine, and didn't highlight the textbox...