I couldn't find a barebone example of this trivial api on the web, so here it is !
Basically, you need to initiate pointer lock mode trough a DOM element that is being clicked. You cant just enter pointer lock mode without user interaction.
Only one element can have "pointer lock rights" at a time, this element is referenced in
There is two events you can listen on :
pointerlockchange: when the pointer lock state changes
pointerlockerror: when an error occurs during a pointer lock attempt
That's it, now go make the FPS (or something else) you where trying to make !