に賛成です

agree with; be in favor of

→ に
phrase Common #4,758
agree with; be in favor of Used to express agreement with a person, opinion, or proposal. The particle に marks what or whom you agree with. 賛成だ is the less polite form.
JA: 私はその計画に賛成です。
EN: I agree with that plan.

あなたのおっしゃるに賛成です

I agree with what you’re saying.

ある程度まで私はあなたの言う事に賛成です

I agree with what you’re saying to some extent.

幾分に賛成です

I agree with you to some extent.

君とトムに賛成です

I agree with you and Tom.

私はこの計画に賛成です

I support this plan.

その法案に賛成です

I support that bill.