Payment Request is sent via Email and will contain a link to a Payment Gateway if setup. You can create payment request via Sales Order or Sales Invoice.
Create Payment Request via Sales Order
Create payment Request via Sales Invoice
Select appropriate Payment Gateway Account on Payment Request. Account head specified on payment gateway will considered to create journal entry.
Note: Invoice/Order currency and Payment Gateway Account currency should be same.
You can notify customer from Payment Request with print format. If customer contact email is mentioned, it will automatically fetch email. If not so you can set Email Address on Payment Request.
Next: Credit Limit