Exchange Hub transport Server will be automatically load balanced with in the Active Directory Site. Hub Transport Servers are load balanced in Round robin fashion with in the Site. If you want to Force Exchange Mailbox server to use specific Hub Server you need to make use of SUBMISSIONSERVEROVERRIDELIST.

Set-MailboxServer -Id:MailboxServer1 -SubmissionServerOverrideList: htserver1,htserver2

Above commands will set MailboxServer to  use htserver1 and htserver2 for mailsubmission. It is normally used for troubleshooting.