Prior to reviewing the instructions below, please see How do I request and API signature? 

Here's how:

1. Log in to your Magento admin panel. 
2. Click Stores. 
3. Click Configuration. 
4. Click Sales. 
5. Click Payment Methods. 
6. Click Configure for your desired PayPal solution and use the following settings: 
   • Email Associated with PayPal Merchant Account - Enter your PayPal email address.
   • Authentication Method - Select API Signature.
   • API Username - Paste in your API username.
   • API Password - Paste in your API password.
   • API Signature - Paste in your API Signature.
   • Sandbox Mode - Select No unless you're using PayPal Sandbox credentials.
   • Enable this Solution - Select Yes.
   • Enable PayPal Credit - Select Yes.
   • Title - Enter your title as you would like it to appear.
   • Payment Action - Select either Sale or Authorize.
   • Display on Product Details Page - Select Yes (recommended).
   • Display on Shopping Cart - Select Yes.
   • Payment Applicable From - Select All Allowed Countries unless you intend to restrict locations.
   • Transfer Cart Line Items - Select Yes.
   • Enable PayPal Guest Checkout - Select Yes (recommended).
7. Click Save Config.

See also:

Magento Forums
Adobe Commerce Developer Documentation
Magento 2 Update

Refer to our preferred Platforms and Marketplaces Directory for addional shopping cart specific information.