'An undeclared property' when trying to create record via Web API

Question

I am getting an error which I just cannot seem to debug. I am trying to create a custom activity entity via custom HTML/JavaScript web resource.

The user clicks a b

Answer 1

After a good night's sleep I realised my error. To set the value of a lookup field, you need to use the relationship scheme name, and not the property name.

Once I changed that, all worked fine.