Hi @Anonymous,
Well, that will depend on which data source you would like to use.
For example, if here we use SQL Server as the data source, then we could enable an auto-generated ID (Primary Key) field, then under PowerApps, use Concatenate function to combine the string with the number autogenerated, to show it in PowerApps:
Concatenate("FM-", text(SQLField))
Things to note is, the auto-generated field under the data source side can't be updated or created under PowerApps.
There is another thread for your reference, for methods to generate GUID:
Create a GUID
Regards,
Michael