While you're waiting for a comment from our ATC colleagues FWIW the only format I have seen from the aircraft side of things for a hand over is (excepting trials) a CPDLC message of
:
"Contact WXYZ" followed by the frequency.
Change over and do the initial contact/check in by voice, then back to the "silent" CPDLC again for direct to's/level changes etc, unless ATC request otherwise.
Hope that's of some use.