VBA Mandatory Fields before submitting worksheet to email address

Looking for advanced help with Excel/VBA. I am creating a catering form that a user will fill out and submit. There are three different sheets in the workbook and the user may only need to fill out 1 or 2 of the sheets at a time. On each worksheet, there is a submit button (that through VBA) sends the completed form to an email address.

In each worksheet, there are mandatory fields. I want to ensure all fields in the specific sheet are filled before hitting the submit button but I am unsure how to code the "once they hit submit, ensure these fields are filled out"

I can do it so that it prompts to fill in the fields before saving or closing the excel document, but I cannot figure out the "beforesubmit" option.

Any ideas? I can provide the document and code if need be