[Resolved] Add coupon field into the link URL

Coupons are applied in the payment popup window. Check the following documentation:

Creating Discount Coupons Using the Stripe Payments Plugin

Ok. Here is a workaround for anyone looking. Add the code snippet below to the file pp-handler.js located in FTP at:

wp-content\plugins\stripe-payments\public\assets\js\pp-handler.js

jQuery(document).ready(function(){
        var url = window.location.href;
        var name = 'coupon_code';
        var results = new RegExp('[\?&]' + name + '=([^&#]*)').exec(url);
        var thiscode = decodeURI(results[1]) || 0;
		var couponcode = jQuery(document.getElementById("coupon-code")); 
		var applycouponbtn = jQuery(document.getElementById("apply-coupon-btn"));
    
    
        var customfieldsdiv = jQuery(document.getElementById("custom-fields-cont"));   
        if(customfieldsdiv.length && customfieldsdiv.val().length){
         customfieldsdiv.prepend("<div class='pure-u-1'><legend>Customer Info</legend></div>");
        }
        couponcode.val(thiscode);
        
       setTimeout(function(){ 
    
        if(couponcode.length && couponcode.val().length){
           applycouponbtn.trigger("click"); 
        }
         
       }, 3000); 
    
    });

Now just add the param to the end of your product link: &coupon_code=MYCOUPONCODEHERE

And of course replace MYCOUPONCODEHERE with a working coupon code set inside of Stripe payment options.