Yeah it still functions as a normal war so you should be able to call in allies as you would normally. Because it's likely that you won't be an elector as Sweden, it's very likely you won't be the league leader once the event fires. So you'll probably have to wait until the protestant league leader gets into another war, which temporarily disqualifies them for league leadership. The actual mechanics on what determines who the follow-up league leader is unclear (to me at least), but with luck it'll be you (rumour has it is weighted in favour of the player-controlled Sweden but idk), but it will only be you so long as the previous league leader is at war, so declare fast before it switches back again. So it's a very awkward time pressure and a pretty luck-dependent achievement imo.