How can I trigger an external newsletter sign-up form to send a welcome email on Shopify?

Hello.

I would like to know if there is a Shopify app, a platform or a way in which I can integrate a newsletter sign up form that I have in my website to my Shopify store emails.

What I would like to do is to automatically trigger a ‘Welcome’ Shopify email (which I have created in my Shopify Marketing campaign) when a user is subscribed to the newsletter from that sign up form that is outside Shopify (because the front-end of the store is hosted outside of Shopify). That sign-up form is currently showing up a seconds after the Homepage of the website https://www.itomlondon.com is browsed.

The Shopify welcome email should be sent to that specific user that just got subscribed in that moment.

I have considered using the Shopify Flow app, but apparently it is only available for ‘Shopify Advanced’ and ‘Shopify Plus’ plans (and my store uses a ‘Basic’ plan).

I have already contacted technical support but I am only being suggested apps that can’t help me create or integrate a sign-up form that is hosted outside of Shopify’s environment.

Thanks,