I am building a print on demand site and i need to embed this code <link rel="stylesheet" href="https://use.typekit.net/plq2mxn.css">

Hello Dee,

This article looks like it can help you - Tech in Check: How to incorporate CSS into your Wix.com website
If not you can request this feature here - Wix Editor Request: Accessing the CSS Code (Style Sheet) of Your Site | Help Center | Wix.com

Best,
Majd