on board

I'm on board. I’m so on board. （計画に対して）賛成です。

Are you on board? 賛成？

be on board 船、飛行機に乗っている状態

get on board 乗る気になる

He's not on board with the plan.

I'm not really on board with going to Disneyland.

I hope he gets on board soon.

How can we get him on board?