webcal links not opening correctly in outlook

I create a webpage with the following content
<a href="webcal://www.mydomain.se/mycal">TEST</A>

When I click the link, outlook opens and tries to add a calendar, but sets the URL to

//www.mydomain.se/mycal

It also complains my url is incorrect.

I would have expected it to insert a link like https://www.mydomain.se/mycal

Is this an issue with outlook, or am I doing something wrong?

Changing the link into
<a href="webcal:https://www.mydomain.se/mycal">TEST</A>

will make it work. But this seems like an incorrect url and might break other integrations, I guess.

Any ideas?