Add a 'host name' to my iMac's?

Hello

When I look on my routers DCHP tables, I can see my two iMac's but there is no 'Host Name' data next to the IP Address (there is with the various other devices).
Where to I tell the iMac what 'Host Name' I want it to have in order for it to report this back to the router?

 

Thank you, but I have this configured already, but the name allocated is not showing in the router table.
Do I need to enable 'dynamic global hostname' perhaps?

 

yes, that worked! I did not see it earlier because the router need a power cycle to get the new values.

Thank you!