I thought about the question a lot yesterday and found a solution that might work.
I roleplay a lot so I was trying to find a solution that is based on the formal declaration of war, without already destroying half the offensive potential of my enemy.
1.Diplomacy
make a demand (like a province you want + money + tribute) + accept or we will attack
if you are unlucky they agree

, but if you make a complete unrealistic demand they most likely wont, if they do even better, destroy all buildings in the provinces you get and let them rebel...
2. block a port with a one ship fleet
that triggers the formal declaration of war
you can however retreat the ship instantly now, you are at war now and neither you or your enemy looses anything.
3a. lets get it on
3b. the cowards negotiate for a ceasefire. you can make another ridicolous demand if you want
though I dont know if that has any consequences for your reputation.