Empress Maria Theresa assignes the authority of co-ruler to his firstborn son, Joseph II pg. 1.

Empress Maria Theresa, after the death of her husband and co-regent, Francis I on August 18, 1765, assigned the authority of co-ruler in the Hereditary provinces, in the Kingdom of Hungary, and in all the provinces of the Habsburg Monarchy, to his firstborn son, Joseph II, Holy Roman Emperor.
On the decision, the Royal Locotential Council informed the Hungarian authorities through the medium of the Hungarian Royal Chancellery in Vienna, ergo the empress transferred the rights without parliamentary approval. A law passed in 1741, made it possible to her to do so (1741 Act IV.).
From this point, a period of nearly fifteen years of co-regency began, during which the Holy Roman Emperor was responsible for military affairs of the Habsburg Monarchy, but in all other cases, the final decision was made by Maria Theresa.
Reference Nr.: HU MNL OL C 13 1765. szeptember 21.