How to add a B Flat Tenorhorn to Score?

Tenor horn is not listed in the new score wizard but you can create it by making any changes you need after the score is created.

Start by picking a similar instrument in the "new score" wizard. For example if you want a concert-pitch, bass clef staff pick Brass -> Baritone.

If you want a transposing, treble staff pick Brass -> Bb Trumpet then after the score is created right-click on an empty part of the staff and choose Staff Properties and change "Play Transposition: Major Second + octave 0" to "Play Transposition: Major Second + octave 1".

To change the name in front of the staff, double click on the text.