When applying boosts generally they are applied before the action starts and run for duration of the action. My understanding is that if you send to POP or to Reinforce, and you have applied offensive and defensive boosters, when the troops arrive they will have those boosts, and they will remain in effect until the troops return to the town.

When sending to an Onslaught, this is a two step operation. You send to Os to fill and then the host sends. If I boost a 10 min 45% offensive booster and send troops to the Os, does the boost last until after the host sends them?

Or must I send to OS and wait to boost before the host sends the Os - this doesn't seem correct since the hero and troops have already left my castle? 

Or if I send to an Os do I have to boost long enough to ensure the boosts are in effect until the Os is filled and started?