Can’t get into startup manager

We have an early 2009 iMac that I want to convert to a Linux machine, since our os is so old that some webpages and many apps are no longer working. We’re using an external display through the mini display port because the native display stopped working a couple of years ago. No matter what I try, I cannot get it into startup manager in order to tell it to boot from a flash drive. I’ve tried unplugging all extraneous externals, I’ve tried disconnecting the power for a minute, I’ve tried different keystroke combos - nothing seems to have an effect. Is it because we’re using an external monitor? Is there a way I could navigate through the startup menu blindly, without a working display?