The seatbelt key bind, button in the F6 vehicle control menu, sound effects, and indicator light within JG HUD will still work when using a third party seatbelt integration!
Within config.lua, set Config.UseCustomSeatbeltIntegration = true. Ensure that Config.EnableSeatbelt = true, or it won't work.
Head to framework/cl-functions.lua, and find Framework.Client.ToggleSeatbelt. There are two parameters, vehicle & seatbeltOn. seatbeltOn will be equal to true if the seatbelt has been enabled, otherwise it will be false. If you have multiple exports, one for enabling the seatbelt and one for disabling, you will have to use seatbeltOn in an if statement; for example: