Marpurg's prelude to "Der Herr ist mein getreuer Hirt" is a nice short prelude. It consists of two halves, the second of which is nothing more than an harmonisation of the choral melody. The first halve is the more interesting one. It is conceived as a sort of pastorale. The hands play a two part texture that flows softly and elgantly above long held notes in the pedals. In my performance I only play this first halve. The score contains of course both halves.
The recording was done with the Hauptwerk software and the sampleset, made by Sonus Paradisi, of the Schnittger organ in the St. Martini-kerk, Groningen (https://www.sonusparadisi.cz/en/organs/netherlands/groningen-st-martini.html).
Score available here: http://partitura.org/index.php/friedrich-wilhelm-marpurg-der-herr-ist-mein-getreuer-hirt