You can use the Quick Setup on the OS3's web interface to set up a connection to a DMR network. After that, read the "C4FM Notes" section on the Homebrew/MMDVM connector's user manual page to find out how to start quick calls to link/unlink TGs:
https://manuals.sharkrf.com/openspot3/en/homebrew-mmdvm-connector.html#c4fm-notes