One search form and field, multiple submit buttons

I have done a few Google searches on this topic but keep finding the opposite of what I need. Here’s what I’m trying to accomplish:

My custom search form will have one search field, where a user can type anything, but there will be 4 different search submit buttons. Each button will say “Search by…” and will only search the query in the custom field indicated on the submit button they click.

So how am I able to specify the custom field in the submit buttons? Thanks.