How to create a bios_grub partition during installing openSUSE?

When installing openSUSE Tumbleweed, I get a red warning before the last confirmation: Boot from MBR does not work together with btrfs filesystem and GPT disk label without bios_grub partition. To fix this issue, create bios_grub partition or use any ext filesystem for boot partition or do not install stage 1 to MBR. The warning is right, installed like that openSUSE can't boot. As far as I understand my machine is probably in EFI mode, because YaST creates something that looks like GPT to me (at least in the graph visualization of the expert partitioning I can see GPT under both /dev/sda and /dev/sdb). What may be relevant is that automatic partitioning creates a BIOS boot partition as /dev/sda1. Apparently this is not enough, neither is setting /dev/sda1 as the custom boot partition instead of MBR on at the boot loader settings, so how do I turn this one into a "bios_grub partition" (whatever that is)?