Hi Darzouras,
Yasaman’s here from the Subify subscription app.
You can use this permalink for the mentioned requirements:
https://[MY STORE].myshopify.com/cart/clear?return_to=/cart/add?items[id]=[VARIANT ID]%26items[quantity]=1%26items[selling_plan]=[SELLING PLAN ID]%26return_to=/checkout?discount=[SHOPIFY DISCOUNT CODE]
As an example, this is the created permalink for our demo store:
Password is: 1234
Hope this will be helpful.
@amanda3e , guess you had the same question.