At the age of sixteen he joined the Bar Confederates, and served as a secretary to Michał Wielhorski, accompanying him on his diplomatic mission to Paris, France, in 1770. Zajączek stayed in Paris for several years, and in 1773 he met one of the major leaders of the Confederacy, Casimir Pulaski. Falling under Pulaski's influence, he left Wielhorski's service, and accompanied Pulaski on his diplomatic mission to the Ottoman Empire in 1774, where he witnessed the Ottoman defeat at the Battle of Kozludzha on 20 June. Returning to France later that year he wrote a hagiographic biography of Pulaski in French. In 1775 he got an officer (junior lieutenant) position in the hussar regiment of the French Army, although after several weeks he abandoned this position to return to Poland. Thanks to the support from the magnate Sapieha family he received a post of an aide-de-camp to hetman Franciszek Ksawery Branicki.