To Agree, To Approve, To Support
する verb
The 賛成 portion means "agreement", "approval", or "support", so the verb version would also be to agree, to approve, or to support!
さんせいする
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
それがどんな汚い手を使ってでも勝たなきゃいけない試合だってことには賛成するよ。
I agree that it’s a game we must win by any means necessary.