secret key in script tag url

I need to add secret key in script src url, which I add in custom code header from settings->custom_code.
e.g.
I need to call script as following

How can I do it. In above example I can’t pass “$myKey”.

Custom code does not communicate with the page’s Velo code. Also, the wix-secrets-backend API is only available in backend code and won’t work on page code.