If my visa comes after my departure, how do I activate it in Canada?

[SpecialKayScotland]

Hello everyone,

I was wondering if someone could tell me how I can activate my IEC Working Holiday Visa if I've already arrived in Canada?
I leave in 1 week and should be receiving my letter of invitation any day, but should I have left and already arrived in Canada, do I have to leave the country and come back in through customs to have it activated?

I'm from the UK by the way.

Any advice would be amazing.

Thanks