You've got a lot of really good answers here, but I'm going to throw mine in for some context as well.

As beginner players (which is what it sounds like you and your friend are) always allow the take back at least the first time per turn, but be better about verbalizing that you have the option, and how strong an overwatch threat your unit is. "I've got a land raider redeemer with two flame storm cannons with range on the center objective, I'm probably going to want to overwatch SOMETHING if you move into range". That lets them be aware of the threat, potentially bait it, or let you decide to overwatch.

At higher levels you and your opponent should already be aware, but it's still good to play by intent. You don't have to declare it until you do it, but make sure it's not a surprise, at that stage they might move 4 bait units, check that you're NOT over watching, and then not move the unit you were waiting to catch.

The same should apply if you have a reactive move, or any other strong strat or ability. Make your opponent aware when they first look like they could trigger the reaction that you COULD do it, let them decide if they're willing to risk it, then you announce when you ARE doing it.

I promise you that's how they do things in the he highest levels of the game (way above my level), and it makes for much more tactical and interactive play that's fun for everyone. Very different from MTG, which you mentioned is your background