Why am I facing a CORS 301 error with admin APIs on my domain?

[ravi45engineer]

I have a custom flow where If a user clicks on a custom button on a marketing pop-up, a custom tag should be added to the user profile. I am using admin APIs for the same but there's a CORS issue that I am facing in this flow. This is working perfectly fine when using through shopify URL (xxx.shopifyapp.com) but when trying the same flow from mydomain.com I am facing 301 CORS issue.

Similar issue is happening on /cart/add as well. In shopify domain /cart/add is giving a response back but on mydomain.com this API doesn't return any response even if the status code is 200.

Do guide me on this.