Tradition has it that while praying for ‘forgiveness of the sins of mankind’ St. Francis received a vision of our Lord and his mother, in which the Lord asked, “What do you wish me to do to help poor sinners?.” Francis replied that he wished to obtain full pardon for all who visited the church of the Portciuncula and made a good confession. Our Lord and his mother were pleased to grant such a request and immediately the saint marched off to the Pope and begged for the coveted indulgence. Read the story here.
This indulgence is listed in the 1968 post-conciliar document The Enchiridion of Indulgences (Section 65: Visit to the Parochial Church). This would include The Cathedral Church of the Diocese; a Franciscan Church, or a church where Franciscan third order members meet.
The ususal conditions apply; sacramental confession, holy communion, and prayer for the Holy Father (Our Father and the Apostles Creed). The Apostolic Constitution on Indulgences is contained in the Enchiridion and is the current authoritative source on the subject of indulgences. (Download pdf here)