Enable refresh to support authentication through custom HTTP headers

[peterbb]

Lots of 3-party data sources exposed as APIs requires additional information to be posted in the HTTP headers. This is already supported in PowerQuery, but not in the data refresh feature of PowerBI.

[bweissdev]

Custom behavior would be lovely... or at least a bit more flexibility

[fbcideas_migusr]

Adding support for API keys to be saved in the service in similar fashion to how they are in PowerBI Desktop is a key use case for this.