Microsoft PowerApps Custom Connector

I need to create a custom connector to a free api for converting UK OSGB36 grid references to WGS84 lat/long co-ordinates.

I need someone to provide step by step instructions on creating the custom connector in PowerApps, what settings to enter in which box etc.

This must be in Microsoft PowerApps nothing else, you will need your own environment to test it in first and then tell me the steps to copy it. The api details are here https://www.getthedata.com/bng2latlong

Alternatively, I am happy to consider someone providing me a canvass app with manual calculations performed within the app so that I can replicate it into the app I am building, this would enable the calculations to be performed off line which would be helpful.