When you create a test business account in the Sandbox, an API signature is generated for that account automatically.

Here's how to find it:

1. Go to the Sandbox Accounts section of your developer account.
2. Find the business account whose credentials you're checking and click the 3 dots at the end of the row.
3. Click View/Edit account.
4. Scroll down to the NVP/SOAP Sandbox API credentials section.

You can also find your API Credentials within your sandbox account:

1. Go to Your payment API credentials within the Sandbox.
2. Scroll down and click the NVP/SOAP API Integration (Classic) tile.