I have finally found the solution to this (previous thread: How to create a connection to Reddit Using the "Make an OAuth 2.0 request"?) @Michaela I wanted to edit that thread with a link to this one so users see that I have a complete solution to connect to reddit via HTTP Oauth 2.0, but anyway to the solution:
Ensure to use the same redirect URI in your Reddit custom app setup.
In the ‘Custom headers’, an Authorization header needs to be created in this format:
Now, to get the base64 encoded string, you can use a ‘Set Variable’ module with the base64() function (the encoded string will always be a constant so no need to worry) :
This worked for me and I have finally setup an authorized connection to Reddit through make.