If you want a RT horn, use Bar Settings RealTrack button to add the instrument to a specific (unused) part.

If you are writing your own MIDI track for the horns, using the Soloist part for this would work. Using whatever method you like (piano roll editor, notation editor, etc), enter the notes at the particular bar and change the instrument patch to what you desire. I don't know what your level of expertise is but let us know if more help is needed.

R