To obtain credentials, you must be a Gong administrator.
Click Company Settings > Ecosystem > API.
Click Get API Key to receive the access key and the access key secret.
Make a copy of these and use them in the header.
Username = Access Key
Password = Access Key Secret
You will be notified when the key is due to expire so you can renew it.
If you want to publish your app in the Gong Collective, follow the steps in Create an app for Gong and use oAuth for authenication.