given my AD domain as: EXAMPLE.ORG if I have a client with dns name: client1.hq.example.org
which steps do I need before joining client1 to EXAMPLE.ORG domain in order to have client1 inserted in the "HQ" ou?
is it automagical?
do I need some setup on AD controller?
It isn't magic, you just need to pcreate the computer account in the HQ Organizational Unit. It's called pre-staging.
Prestage client computers
https://technet.microsoft.com/en-us/library/cc759196%28v=ws.10%29.aspx
https://technet.microsoft.com/en-us/library/cc770832%28v=ws.10%29.aspx