Pabbly Subscription Billing provides an option to Auto-populate any field of form data through a Query String. So if someone wants the coupon code to be prefilled to the checkout page URL then simply add a query string for the same in the below format - 
		
		
	
	
Follow these simple steps to auto-populate the form fields - 
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a) Add a question mark (?) at the end of the checkout page URL.
b) Add the 
Parameter of the field, followed by an equal (
=) sign & the 
Value that you want to pre-fill in form.
c) For pre-filling, the country and state names use the specific 
Codes like for India as in, etc. 
d) Separate multiple form field parameters with an ampersand (
&).
Note: There should not be any extra spaces in the URL
Now, you need to combine the checkout page URL, form field parameter, and the values that you want to pre-fill for your customer. This way, in the above manner you can generate a URL with auto-populate form fields through a query string. Therefore, share this URL with the customers using any medium like email & messages, etc. So the user will be able to check the link with prefilled values. 
For detailed info, please follow this doc - 
https://www.pabbly.com/subscriptions/docs/auto-populate-form-fields-through-a-query-string/