Here's the asteroid Bennu observed from OSIRIS-REx spacecraft,
Date__(UT)__HR:MN R.A._(ICRF/J2000.0) _DEC APmag
2018-Sep-17 00:00 17 46 40.89 -08 41 51.7 11.64
But so far I can not do the reverse, and observe OSIRIS-REx from Bennu. Let's see what is possible, then as an absolute last resort, read the documentation.*
This is an interesting question. Going to horizons and playing around with some sites to observe Mars I found that in addition to locations on the surfaces of some solar system bodies such as the Apollo 11 landing site on the Moon (otherwise known as 500@301) and observatories in Jupiter Florida and the Luna Cafe in Flint, Michigan:
we can also observe from the solar system barycenter (a mathematical point in space and usually somewhere inside the Sun) as well as the body centers of the Earth, Moon, or Moons of other planets, which don't require surface coordinates.
or even from some NASA spacecraft:
and so some interesting combinations are possible
But I haven't been able to specify centers of major asteroids such as Ceres, Pallas, Vesta, which are physically huge and, have over 200 years of tracking observations. So there must be a method to this madness.
It looks like you can observer from anything that's in the JPL Development Ephemerides (so @705 is "moon #5 of planet #7") or from some spacecraft.
And so as a wise man once said: If all else fails, read the directions!
Which is in this case, the documentaiton which is something that NASA excels at almost always.
In the Horizons output there is a link to https://ssd.jpl.nasa.gov/?horizons_doc#site which says in part:
COORDINATE CENTER (OBSERVING SITE) SELECTION
Once a target is specified, the next step is to specify the origin of the coordinate system, or the "observing point", relative to which the ephemeris should be expressed.
While osculating element tables may be generated with respect to a major body center only, vector and observer tables may produce output with respect to an arbitrary observing site, defined with respect to a major body center.
For the Earth, a list with the locations of 1900+ sites is predefined. The list generally matches that of the Minor Planet Center while providing an expanded list on radar/radio sites (which have negative ID numbers). Station "500" is the geocenter.
For non-Earth major bodies, station 500 also represents the body center. For those major bodies with IAU rotational models, additional topocentric sites may be defined. Spacecraft landing sites are typically predefined on non-Earth bodies.
SPECIFYING A PREDEFINED SITE
There are several equivalent ways of specifying an observing location. The most general form is ...
site @ body
... where "site" is a numeric code or name fragment to match, and "body" is a >numeric major body code or name fragment to match. A list of such major body codes follows later in this document, or type "MB" at the main Horizons prompt in the telnet interface, or send "COMMAND= MB" via e-mail interface.
Here are four equivalent ways of searching for the same Earth location:
675@399 Site #675 on Earth (Palomar Mountain)
Palomar " (observer table only)
OBSERVER & VECTOR TABLES:
If an observer or vector table has been requested, the "@" symbol may be dropped; the Earth will be assumed if an integer like "675" or a name fragment like "Palom" is input. However, if you are trying to specify an observing site not on Earth, you MUST use the "@" symbol for correct interpretation. For example, if an observer table as seen from the Sun is desired, it must be specified as "@10" or "@sun". Specifying "10" only will select the Caussols site.