Assuming that you are building via TKLDev then the appliance will inherit the hostname from the top level build code directory. I.e. if your buildcode is /turnkey/fab/products/foobar then the hostname will be 'foobar' by default.

If you are pre-seeding, then you can set the hostname by setting "HOSTNAME" in the pre-seed file. E.g.: a line that says 'HOSTNAME=barfoo'.

If you want it to be interactive, then you'll need to create a script to do that as there isn't an existing inithook to do that. If you're familiar with python, then you can leverage inithooks to create a new script that sets the hostname. Drop your new script into overlay/usr/lib/inithooks/bin/ and create a new (executable) firstboot script (in overlay/usr/lib/inithooks/firstboot.d/) that calls it. The existing 09hostname firstboot script might be a useful reference? And/or the Confconsole hostname plugin.

I hope that helps. If you need further clarifcation and guidance, please ask.