From HEXONET Wiki
The TransferLock feature exists to prevent unauthorized transfers of domain names to other registrars. If the TransferLock is enabled, domain names cannot be transferred unless they are unlocked.
You can request the the domain to be locked within the AddDomain, ModifyDomain, and TransferDomain command.
The TransferLock can be set using the
TRANSFERLOCK API parameter.
E.g. to set the TransferLock for an existing domain the following command can be used:
command = ModifyDomain domain = mydomain.tld transferlock = 1