Payment Request -how to set Payment URL


Step 1 : I have set Paypal Sandbox Account
Step 2 : I have Sales Order , from where i generated Payment Request
Step 3 : From Payment Request page, the below message is set

<p>Dear {{ doc.contact_person }},</p>

<p>Requesting payment for {{ doc.doctype }}, {{ }} for {{ doc.grand_total }}.</p>

<a href="{{ payment_url }}"> click here to pay </a>

the above message all the variables gets replaces as below in the payment request email received

Dear Amit-Amit,

Requesting payment for Sales Order, SO-00006 for 2000.0.

click here to pay

My question is

  1. how to set payment_url ? as the link generated is [erpnext site url]/None

Payment Gateway Account has this setting