Fitbit is a competitor of Garmin, so if you ask how to use a Garmin device directly in the software of their competitor, then the answer would be that it is not possible. However, you may be able to import activities or other data to Fitbit software, in one of the file formats you can export from Garmin Connect (FIT, TCX, GCX, CSV,...). And you can also use 3rd party tools (like for example https://www.fitnesssyncer.com/) for interconnecting the accounts.
If you asked your question first to Google, you would find threads like the following one, with some more tips for the Fitbit/Garmin interconnection:
