Permalink to a Pre-filled cart in Squarespace

First, a disclaimer: I am building a store for my own products and attempting to learn/re-learn coding in the process. I'm using Squarespace in an attempt at keeping the tedium at a minimum.

I need to make a permalink to a cart pre-filled with specific items and discounts. I found (I believe) the product variant ID's using both the json-pretty method and the Chrome developer's tools. Best I can tell, those are correct. I checked the links, and...I am taken to an empty cart, even with other products and no discounts. So, I see the following possibilities:

• I have incorrect variant ID's
• I have my formatting wrong (I am using https://sitename.com/cart/#{variant_id}:#{qty}?discount={discount_code})
• This can't be done in Squarespace, at least not how I'm doing it

I've been searching and just can't seem to find where it's going wrong. Please help, thank you.