Can you describe the event where Sai Baba requested a Samadhi be built for him?

Yes, this event occurred thirty-two years before his final passing, during a period known as the 1886 Trance. As detailed in Chapter 44, on a full moon day, Baba experienced an asthma attack and decided to enter a state of Samadhi for three days by drawing his life force into the crown of his head. Before doing so, He pointed to a specific spot in the hall and gave a direct instruction, saying, "Dig a Samadhi there and keep me in that place." He then firmly told Mhalasapati to guard His body for three days and not neglect Him, even instructing him to place two flags at the spot as signs.