Prompt for input on record-level custom action

I'm developing a view with a custom action for each record, but I want to be able to have a prompt for one of the variables passed to the function. Right now it looks like all the arguments must come from the record. What if I want to customize a different message that is sent to each record's email address via a custom action? I want to be able to click on a button next to the record "Send Email" and then be prompted for custom input to pass to the email function. How would I do that?

Thanks!