Hi @Hirdesh Baghel ,
first step: Create a registration key: Generate a registration key
second step: register existing VM to a Hostpool: Register session hosts to a host pool
The command line registering needs to be executed "inside" the VM you want to add to the Hostpool.
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten