Solar system bodies which are not included in the "Choose a Solar System object?" drop-down list in the Field Setup section of a Science Goal can be specified using the 'Ephemeris' option in the same list. This lets you load a JPL Horizons file as the ephemeris. Please see the 'Create an External Ephemeris' section of the OT User Manual available from the Science Portal under 'Documents & Tools' for more details on the exact format to be used.
You may additionally want to enter time windows during which the observations can be scheduled by selecting 'Single Visit' in the 'Are the observations time-constrained?' section of the Control & Performance editor of the OT. Here you can specify as many time windows as you want either manually or by reading the possible time slots directly from an ASCII file. Your Science Goal will then be executed during one of the time slots specified.