You can't add text directly to a map. Another option is to drag tokens to the map and then add "tooltip" text to them by typing a label in the chat window (don't press enter) and then drag the label text from the chat window entry area and drop it on your token on the map. This gives that token the label text as a tooltip (hover the mouse over the token to see the text).
Adding text directly to the map is a fairly popular item in the FG wishlist:
https://fg2app.idea.informer.com/proj/?ia=40911 Go vote for it, although it probably won't be looked at until the future migration to the Unity engine.