Using the Usabilla API with Power BI - Creating a "Canonical Request"

I've been able to use Power BI to connect to several services using their API's however the documentation for Usabilla API has got me a little lost!

Specifically the authentication process asks to create a "Canonical Request". Has anyone been able to achive this via Power Query and could give me some pointers?

https://developers.usabilla.com/#authentication