It takes time.
First of all, experts didn’t wait for your forum to ask questions and write answers. They are already active somewhere. So you have to find the different curent places they are, and start with interacting with them here. And I don’t mean that you must come and tell “hey come on my place”. You must first become a regular interlocutor for them.
Then, if they come on your forum and there is nothing interesting, they will not stay. So you have to provide a base content, writing interesting content, and work with your team to create threads. It is the quality of the content and discussion that will attract expert.
And eventually, if you have some success, a strong moderation is important.
Last but not least, is a forum still relevant? (there are plenty). Should you start instead a Q&A? or join an existing one and be visible there? or, become the founder of a dedicated financial Stackoverflow or Reddit instance? There are many things to consider before starting yet another forum on something.
Ah, and again, it takes time.