Set up redirection (or CNAME record) from something.mydomain.com to yourhostedaccount.postaffiliatepro.com
It is important to only setup the CNAME record. Do not insert and/or setup A record (pointing to your account IP), since it could lead to a not functional account in the future because IP might change (because of account transfer to another - faster server)
Note: yourhostedaccount.postaffiliatepro.com is your original hosted account domain
Additional info about setting up CNAME record on various hosing services are here.