This from an enroute doc but if you read the end note re data link I guess whatever format the data link would pass it in is good enough.....
5.4 Meteorological reports
5.4.1 When voice communications are used, oceanic area control centres shall designate, from among the aircraft intending to
operate on the organized tracks, those which shall be required to report routine meteorological observations at each prescribed
reporting point. This designation shall be made by the oceanic area control centre delivering the oceanic clearance, using the phrase
“SEND MET REPORTS”, and should normally be made so as to designate one aircraft per track at approximately hourly intervals.
Note.— When air-ground data link is being used for meteorological reporting during flight, no aircraft observations by voice
communications are required, in accordance with Annex 3, 5.3.1.