If you're planning to integrate APIs with Nebula, you'll have to provide your Nebula Account ID. To find your Account ID, log in to Nebula. Once you're logged in, look at the URL in your browser. You should see a string of characters after ".com" in the URL. That string of characters is your Account ID.
Example - https://cloud.malwarebytes.com/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/