1. Open API section in settings

Log in to your PayPal business account and click on Profile and open your Account settings afterwards.

You'll see the page you can see below. Click on My selling tools and then on Update in the API access row.

PayPal Account settings

2. Request API access

Click on Request API credentials to generate your PayPal signature.

Request API access for PayPal

3. Confirm request

Select Request and API signature and click on the confirm button.

You'll then see your API signature data.
Add this information to your Landingpage. Afterwards your users will be able to make payments using PayPal.

Confirm request for PayPal API access
PayPal API signature