How to use an API key with the HTTP funcitons

I have a web service that provides an API key string.

The "Set API Key" function in the HTTP Client -> Security palette has two inputs, the private and public sections of the key.

Setting the given key for either/both does not work,  Setting my usernam as the public portion also does not work.

The service (Pushbullet) gives this example in their documentaion:

curl -u <your api key here>: https://api.pushbullet.com/api/devices

Any ideas how to make that work?