InvalidMashupConnectionString Error on Custom Connector / Custom API

[Bizualisation]

Hi all. 

Really hitting a wall here so any help is appreciated!

Due to project constraints I need to use custom APIs to access some data. Now, the authentication is handled through API keys which we pass as part of a URL so the connectors authentication type should be anonymous. Web connector doesn't allow anonymous access in the gateway so I made my own connector which seemed to be working - it works fine in the desktop and pulls data through no problem. 

However, when I post to powerbi.com, add it to a on-prem gateway I'm seeing this error message

I can't find too much online about the ConnectionString issue. I guess this means the URL I'm passing to the gateway isn't working for some reason, but I'm really struggling to figure out what the issue actually is.

My custom connector currently accepts 4 parameters (1 of which is mandatory, the other 3 are optional) and the code looks like this:

As you can see it basically asks the user to enter a base URL, and a relativepath (i.e. dynamic portion of a URL) and our two authentication headers.

Finally, I'll point out I'm quite a beginner at this sort of thing, so this might be a really stupid mistake!

Thanks in advance, and if any clarification is required please just let me know 🙂