how to format a SSD for using on time machine?

new TM drives need to be formatted as APFS with GUID partition map.

please see this apple support page: Erase and reformat a storage device in Disk Utility on Mac - Apple Support

If the Macs that you are using with Time Machine are running macOS High Sierra or later, then you should format the SSD for APFS using the Disk Utility.

Ref: File system formats available in Disk Utility on Mac - Apple Support