You can ask them to start working with you on a small part of your idea for which you want them to partner with you too. But I think a better idea is to create

You can ask them to start working with you on a small part of your idea for which you want them to partner with you too. But I think a better idea is to create a new small project. And ask them to work on it for you first. Keeps the real project a bit distanced before you both are ready to commit to a partnership.