How can I integrate Sendgrid API for theme contact form submissions?

[choogoor]

Hello, I need to integrate Sendgrid for my theme contact form submissions. But the Sendgrid API key needs to be stored in the environment variable and sent within a header as authorization in the API request. Since there is no way to have it in the theme, I discovered that I might need a Shopify APP Proxy. But in that way, I can set the API URL but no headers with authorization (API key). 

Is there a way to solve this without creating an app (cause it seems to me it's too much work for a relatively small thing)? If not so, any advice on how to do that with the app?

Thanks in advance.