Hi @vladimir-d,
I think I have the same problem and the same wish as @jfftck, Cal.com limits Mirotalk URL to the mirotalk.com domain. But, like me, he must have a Mirotalk instance on his Cloudron server and can't fill in the address because Cal.com considers it invalid.
And Cal.com's developers don't seem to have decided to change the regex any time soon: https://github.com/calcom/cal.com/issues/16258