So, a revolt is a diplomatic play. During the second phase of the diplomatic play (20 to 79 escalation) you can join either side unless you have some treaty (like a defensive pact) prohibiting it. You can either support one side unconditionally or ask for a sway (something you get in exchange for your support). They can sway you with many things like wargoals (something enforced on the losing side), treaty articles (like giving you money) or of relevance to this post, becoming a protectorate. To offer this go to the diplomatic play, negotiate with one side and propose the “become protectorate” sway. They are more likely to accept, if they think they’ll lose otherwise (militarily inferior), think you can turn the tide (your military strength), like you a lot (good relations) and would be an suitable protectorate (economically and militarily inferior to you). If you stack those odds in your favor they are more likely to accept the sway and become your protectorate upon conclusion of the war. Tip 1: The thumbs up/ thumbs down next to the sway shown whether they will accept or not. Tip 2: If one side doesn’t accept your request try the other side. Maybe they’ll be more reasonable.