The 1931 Constitution of the Kingdom of Yugoslavia asserts the right to form associations under Chapter II, Article 13. Slovenia gained independence in 1991, The 1991 Constitution of the Republic of Slovenia covers freedom of association in Chapter II, Article 42.

Constitute Project. Republic of Slovenia 1991. https://www.constituteproject.org/constitution/Slovenia_2013.pdf

The Constitution of the Kingdom of Yugoslavia. 1931.https://royalfamily.org/wp-content/uploads/2014/01/constitution.pdf