What
@Attack Chimp said. You need to be already overheated.
As for the exact heat threshold and damage buff figures, more tests would come in handy I think.
Old test results (just off the top of my head) used to show 10% ish bonus at 100% heat and 30% ish cap at very high (above 150% 170%?) temperatures (it was made by
@Crimson Kaim IIRC). According to more recent tests it's no longer the case. The conventional wisdom at GCI (Galactic Combat Initiative discord server) is - if below 100% heat you deal the stated damage for the weapon after engineering blueprint (plus maybe some insignificant damage buff at heat levels close to 100%). If above 100% heat you deal an additional 60% (total 160% damage) of the stated damage for the weapon after the engineering blueprint. For PAs. I'm not sure about other weapons (beams etc).
It's well worth it to join the GCI discord if you have further questions BTW.