I'll suggest a couple of ways that work for me in QA-testing (inc. with 20.04.2 media)
Fn keys
Use laptop FN keys to switch between mirror,internal,external,internal+external etc. I've not had issues with this, however it's likely firmware specific.
Indicate which cursor you want it using the mouse
Try using the mouse to indicate/control which screen you want the installer to start on... ie. I'm suggesting clicking on the start installer option & very quickly move the cursor to the monitor you want the window to appear on. It works.
However on fast boxes this maybe harder to take advantage of than slower hardware. I use it regularly on boxes with landscape+portrait monitors in QA-testing so I don't have to use the "Monitor.Settings" to make the monitors match my setup and I'm not stuck using a the sideways (portrait) monitor (installs without me doing this are tested for of course too), or on the box besides me where I want it on the display further away as the mostly-white-install background is very bright.. This works.
Monitor Settings
If I'm doing a QA-test where I've already attempted the prior method (a faster box usually), and I realize i'll waste resources to repeat trying it.. I just adjust the positions of the monitor using the LXQt Monitor.Settings. The Fast Menu will likely be fastest (click the drop-down to the right of the following picture)
The manual can be read at https://manual.lubuntu.me/stable/3/3.2/3.2.10/monitor_settings.html