Your Username is the username you added when you created your Application. This is not to be confused with the APP name which is just the name of your app.
You can view both the username and app name on your dashboard :
The username is what you use for authentication when making a request to our API.
Your API Key is also used for authentication when making a request to our API.
On Sandbox,the username is always sandbox, you can view this from the Sandbox dashboard as well.
Here's an article on how to generate an API Key.
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