How to call a HTTP request to an external api using base64 encoded signed payload

Summary:

We need to send a signed payload (a combination of a signing algorithm, JSON payload, and signature) as a base64 encoded string to an external API using an HTTP POST request. I've tried several methods without success. When I pass the decoded payload, it fails due to different characters in the signature. When I pass the payload as binary-text/plain, I get an invalid logical path error.

The external API requires the JWS Compact Serialization format, where the entire JOSE Header must be a JWS Protected Header. This header describes the JWS digital signature or MAC on the header and payload. Both the JWS Protected Header and the payload must be converted to UTF8 and Base64-URL encoded.