Dual-boot Windows & Debian - how to install Debian to /dev/mapper ?

I'm a newbie to Linux & I followed this Ubuntu guide in my attempt to dual-boot Win 10 & Debian 12 on one drive, and to encrypt Windows with Bitlocker and Debian with LUKS. The guide uses sgdisk to manually create the various partitions needed.

Everything went well until "Phase 4" in the instructions. I'm not sure what the equivalent instructions are for Debian's installer - any suggestions would be appreciated. ("Phase 4" instructions says I should tell the installer to use /dev/mapper/* locations for / and swap, but the Debian 12 installer doesn't list /dev/mapper locations when I select the Manual partition method). I also tried Debian 11, with similar results.