In 1945 Hamidhan was chosen by the Japanese as a representative of Borneo at the Preparatory Committee for Indonesian Independence (Panitia Persiapan Kemerdekaan Indonesia, or PPKI) in Jakarta (renamed during the occupation). Before leaving, he spoke with local figures regarding their hopes and aspirations for an independent nation. When Indonesia proclaimed its independence on 17 August 1945, Hamidhan was one of the witnesses. He later wrote "Sekitar Mukaddimah UUD 1945: Untuk Memperlengkap Kebenaran Sejarah" ("Regarding the Constitution of 1945: To Complete History's Recordings"), which dealt with his experiences around the period and decried claims that there was conflict over parts of the constitution relating to God's role in the country.