The recommended option is to use the URL Tokens or Encrypted URL Parameters, and NOT the URL Parameters option.
The URL Parameter options parameters in the URL to control the defaults and behaviour of the online checkout form. This can include which Merchant Facility and Payment Form are being used.
See procedure Which parameters can I use with the various URL Parameter options for a list of parameters that work with Dynamic URL Parameters
1. Example 1: Connect your payment form to your Merchant Facility
1.1. Locate your Merchant Facility record ID
1.2. Place your Merchant Facility record ID into your URL with the parameter
2. Example 2: Place default contact details into the form
2.2. Locate your Contact's record ID
2.3. Place the record ID with the parameter (cId) into your URL
Default values are shown in the payment form.