How do I install 20.xx in safe graphics mode?

Ubuntu runs on an older iMac in "safe graphics" mode from a USB drive.  However; when I run the installer and reboot from the HDD, the screen goes black and the PC is unusable.  My solution to date involved testing & installing Fedora using "safe graphics", but the end user prefers Ubuntu. I assume it's possible to edit "grub.cfg" to include a "safe graphics" installation choice if the correct command string was known?