Primestart is a Yamaha term for any automatic fuel enrichment system. They are not all the same.
Primestart on an EFI F350 or the latest XF425 direct injection model is completely different than Primestart on two stroke models.
https://www.yamahapubs.com/ for the owner's manual. It is the service manual that goes into detail with respect to adding an optional Yamaha temperature sender.
The existing thermostat cover is removed and replaced by one that has a port into which the temperature sender/sensor is installed.
Presumption being that you are referring to the use of a Yamaha standalone temperature gauge.