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1. Log in to the PayPal sandbox at www.sandbox.paypal.com.
2. Click the settings icon at the top of your PayPal account page and then click Account Settings.
3. On the Account access page, click Update for the API access item.
4. Click Manage API Credentials in the NVP/SOAP API Integration (Classic) section.
5. Select Request API signature. Then, click Agree and Submit.