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Log in to Admin Panel: Access your admin panel by logging in with your credentials. 
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Navigate to Subscription: On the left side of the admin panel, locate and click on the "Subscription" option. 
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Access Payment Settings: Within the "Subscription" menu, you'll find the payment settings. Click on it to proceed. 
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Enter PayPal Merchant Account: In the payment settings, input your PayPal Merchant account details. This typically includes your PayPal email address or merchant ID. 
You may follow this link for Paypal documentation :
https://www.appinvoice.com/en/s/documentation/how-to-get-paypal-client-id-and-secret-key-22
Screenshots :
Admin panel: https://prnt.sc/wljdas
Paypal: https://prnt.sc/wljeua