There is a company which has many offices, every office has a specific name, for example, "Mercury", "Earth", "Sun" etc. How do I say?
" See you later on Mercury"
or
"See you later at Mercury"
or
"See you later at the Mercury"
"See you later on the Mercury"
You can use "at" or "in" with an office.
"At" is used with a place or location, "in" with a building (or anything you can physically enter), so there is a choice depending on whether you're more interested in what happens at the place or in the building itself. "At" would include associated facilities (car parking, hut in the garden, pond, etc) and could be used of a multi-building office or preceding a term like "Headquarters". "In" usually means physically inside a specific building and would be more appropriate if if buildings are identified by description rather than name ("in the HR building", "in the main building", "in hut 325") or are emphasising that you'll be inside not waiting in the parking lot. With some places like "at/in sector 7G" it is a judgment call, so you'd just copy other people.
It doesn't matter if your office is called Kingdom House or Mercury, the grammar is the same. "See you at Kingdom House" or "See you in Kingdom House" or "See you at Mercury" or "See you in Mercury". If your office is called something very weird you can (for clarity) call it something like "Mercury Office" or "the Mercury office", or say "see you in Mercury, the office". But if you work at a company that is sending things to Mercury every day, it is probably sufficient to say "Mercury".
An office is a location for which we use "at". One is "at the office", or "at Mercury". There's a real famous record company called "Mercury Records", and if one were to go there, one could say they were "at Mercury". Even though Mercury is a planet's name, one is not referring to the planet, so not "on Mercury". "On Mercury" always implies "on the surface of the planet Mercury".
We name based on the type of object, not its name. We go to the bar, even when that bar is called "Arthritis", for example. Normally, no one can "go to arthritis", but the place is a bar, and you can go to a bar, hence "go to 'Arthritis'".
The best way is to incude quotations around the name of the place, and through this method, one can avoid the confusion of offices named after planets, and just anything named after a weird thing in general.
If you're talking about an office on a planet named "Mercury" (I assume you're writing a science-fiction novel), you would say:
See you later on Mercury.
See you later on Earth.
See you later on the sun.
If you're talking about meeting someone at a club, bar, or restaurant named "The Mercury," you would say:
See you later at The Mercury.
If you had a spaceship named "Mercury," you could say:
See you later on Mercury.
See you later on the Mercury.
