Need help in editing popup form

[snufflebum]

Hi guys,

I am using the Toyo theme and my theme has a popup form that allows the user to subscribe to newsletter for a discount code.

I want the popup to display a message that says "Thank you! You have already subscribed" if they already subscribed to the newsletter using their email ID.

How do I achieve this?

My store URL: snufflebum.com 

Pw: bohtro