...
- Your API credentials are different from your MailUp account login credentials.
- You can obtain these credentials by logging into your MailUp console, as described below.
- You will also need to authorize your application by adding its IP address(es) to your MailUp account.
REST API
The REST API uses a completely different authentication method, and Used by these plugins: Salesforce, Zapier, Drupal Commerce
- If you are using an application that has been integrated with the REST API: you do not need to obtain separate API credentials.
...
- You will simply need to enter your usual MailUp account credentials.
...
- If you wish to integrate your application with MailUp using the REST API, get started here.
Credentials for SOAP and HTTP API
...
Log into your MailUp admin console and look at the browser address field. It will look something like abcd.emailsp.net or a4h4i.s21.it (or alike).
API User Name
...
1: Go to Settings > Account settings > Developer's corner. On the Developer's corner page, click on Web services. 
2: The WS User Name is your API user name. It starts with the letter "a" followed by your numeric account number.
...
For your internet and application security, enter a strong password (6-8 characters long, case-sensitive, include including special characters) in the password field and click on Save.
...
Debugging
The debugging option is for mostly for internal purposes. It enables more detailed errors in certain scenarios (e.g. during import). Typically, it should remain unchecked (disabled).