I'm a recent Windows to 2021 MacBook Pro convert.  Not knowing it was impossible, I managed to stumble on a method using Disk Utility.

- Launch Disk Utility
- Erase the disk and create “Time Machine Backup” volume
- Notice that no option to limit size is available.	
- Delete the Time Machine Backup volume just created.
- Add a volume named Time Machine Backup and supply size constraints.
- Add additional volumes with or without size constraints.
- Launch Time Machine and Select Disk ->Time Machine Backup
- Here's what I ended up with.  Seems to work.
[![Disk Utility][1]][1]


  [1]: https://i.sstatic.net/6qzqX.png