To link MMR go to fitbit.com/apps click on MMR and follow the prompts.
You have t9 go t9 a sight on MMF and ask it to connect to Fitbit, then tell fitbit this is ok.
As for Garmin i don't think they have setup a link to Fitbit but you could ask garmin. Note third parties are responsible to set up their link to fitbit.
One way it might be possible is through IFTTT, that would be something you will need to look into.
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