There is a way using the azure portal.

Go to the API Management service.

In the side nav under the APIs dropdown. Click on the backends menu item. Create a new backend to match your api that you want to turn off SSL cert validation for. Before creating the new backend, there is an advanced dropdown. Click it and uncheck the checkbox labelled as "Validate certificate chain".

I could not find this in documentation anywhere or on other answers. So I am adding it here. It may change in the future.