Intercept data for SaveButton on Modal

Hi. I'm really struggling to figure this out and it feels like it should be simple. I have a modal using the useModalForm hook and that's working fine, when it hit the save button it makes a request using the fields on the dialog. So far so good.

Now I want to be able to manipulate some of the data before it gets sent to the API POST. Is there a simple way to do this?

Thanks.