the notice is because of the products/articles, which can’t be read in the new user area. Thats the reason it is not working for you (I’m the developer, so I know it )
An approach could be the following:
First: Create social media image by fetching designs via api and putting them on a shirt mockup including your other images like your logo… You might have that already, as I can remember, right?
a) This script also creates a link including the designId and your default product (e.g. always default gray shirt) and deeplink to the spreadshirt designer. It could be an embedded spreadshirt designer (sketchomat/tablomat https://spreadshirt.github.io/apps/sketchomat) where you have a lot of control or it could be the default speadshop designer (https://www.spreadshirt.net/forum/t/deeplinks-designer/2040/16), which every shop has.
b) Manually link to the corresponding product in your shop.
What do you think? Would one of the proposals solve your problem?