Hook: Floor Small (Green Square)The Office Chair decoration fits into a small green square floor hook.
Hook: Floor Medium Narrow (Blue Rectangle)The Office Chair decoration fits into a narrow blue medium rectangle floor hook.
How to Get an Office Chair Decoration
Cartel Pack Decorations
The Office Chair decoration can be dropped from Gatekeeper’s Stronghold Packs, which were released on the Cartel Market and bought with Cartel Coins. Cartel Packs, including the Gatekeeper’s Stronghold Pack, include a random selection of items from the Gatekeeper’s Stronghold collection of items. You aren't guaranteed to get an Office Chair decoration from a Gatekeeper’s Stronghold Pack, but it is one of the few ways to get an Office Chair deco apart from buying it on the GTN from another player.
If you're looking to get a Office Chair, the easiest way to get it is to head to the Galactic Trade Network (GTN) in SWTOR and buy it with credits from another player. If it's very rare and not available on the Cartel Market, you could try buying from the GTN and hope you get it.