Time Machine offers to erase disk data (format), will it delete all my partitions?
Look at Erase a volume under EXAMPLES in the diskutil man page.

diskutil eraseVolume HFS+ UntitledHFS /Volumes/SomeDisk


In this case the command line you'll probably want to use is:

diskutil eraseVolume HFS+ BackupMac /Volumes/BackupMac


However check that the command line is formatted as needed before executing it.

I tested this on a 2 volume disk and it only touched the 1 targeted volume leaving the other intact and the target reformatted HFS+, which is what you want for the Time Machine backup.

Note: It may need to unmount all volumes on the disk so make sure nothing is open on the other volumes before hand.  While it may unmount the other volumes not to worry as it will only erase and reformat the target volume and remount upon completion.