If that's ordinary steel on the transom, it needs priming and painting before antifouling or it will corrode badly. (You probably know that - sorry)
Most antifoulings contain copper these days I think. Copper and aluminium (engine) sizzle in seawater and it's the aluminium that disappears.
When I antifouled mine, I left a couple of centimetres gap around the engine bracket. End of season there was a little growth on the bracket and trim cylinder, not too hard to get off, and no corrosion. The anode on the engine bracket erodes much more quickly than before and will have to be replaced next season.
Just about to antifoul again before launching. Will use International, but avoid 'Dover White', because it's grey not white - and it turns greenish in the 'splash' zone.