I am witnessing a strange problem in WordPress which automatically removes 'http' from my sub domain link. I don't know why this is happening.
Note that my domain is https while it's subdomain is http.
My WordPress is installed in my main domain. When I add the below url of my sub domain to any post:
Then it automatically converts it to:
See that the http is gone from the URL.
Also see this below gif image which lists this problem:
Why this is happening and how to solve this problem.