Does anyone know if it's possible in Shopify to encode form data into Json and pass it in RESTlet URL with authentication?

Where in Shopify is the form data being captured. On the storefront or an app. The online storefront you wouldn’t be able to secure you NS credentials. Shopify App would be possible.

@JohnnyC it's on the Shopify Storefront. In that case, could we pass the data to an external server where an app like a PHP could take the data and handle the encoding and authentication?

Yes was going to suggest that as an alternative as well. Creating a Shopify app and using an application proxy would function similarly with the benefit of HMAC sha256 message validation so you know the request was coming from Shopify.

right, thanks @JohnnyC