Can't save dataflow - dynamic data source

Hi 

I've built a function in desktop that I would prefer to migrate to a dataflow, but although it works in desktop it won't save as a dataflow on the service.  The function uses UK Companies House APIs to get basic company data after passing it a company registration number (with the company being dynamic as a separate query loops through a table of companies and calls this function).

The error message and the relevant function M code are shown below:

From what I can find online it may be something to do with it being a dynamic URL (and I have a separate fuction which is dynamic and causes no problems). I tried experimented with using different combinations of RelativePath and a Query parameter but didn't have any luck. 

Any ideas?

Thank you