Instagram and TikTok do not let a website compose your post. Anyone who puts a dead glyph on a product page is lying. We built a desk instead: the mockup, your line, copy and download.
Open Share from any shirt. Pick the color. Copy the image if you are on HTTPS. Download it if you are not. The line already has the product link. You already know your accounts.
Later draft: what to write in the line so it does not sound like “check out this product.”