If you are storing these tickets in a database; when submitting a new ticket, you could possibly get the max value from the database and perhaps +1 to that. However, as FM-00000 would be a string, you might have to try split it by - and get the right side of it, to add a 1 to it.
For a more simple approach, you could just have a column for Type which could be FM or AM or w/e and another column called Ticket_Id which is just a number. This way you can get the max number easily and also add to it easily.
Hope this gives you more of an idea on how you might want to tackle this problem.
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 Concatenatefunction to combine the string with the number autogenerated, to show it in PowerApps:
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: