Is it possible to save an email address even if the form is not submitted

I am building an easy email signature configurer and would like to store email addresses even if the user doesn't complete the design form? 

Based on what you have shared, I'm not sure if you want to store the email addresses temporarily or permanently. You can use the localStorage for saving and recovering any information the user provided while filling in the form, in case they decided to come back after abandoning it.

But on the other hand, if you want to save a list of email addresses to act on later. For example, sending a reminder email to users in question about filling in their abandoned forms or something. Your best bet would be to create a new model and a controller to build an API endpoint that can be called by your JavaScript code using AJAX whenever the user fills in their email address.

yes, perhaps whenever the email address field loses focus, it sends it off the endpoint, perhaps validates it there, and then persists it somewhere. (perhaps in ConvertKit or Drip via another API call)