Catalyzes the conversion of 3-hydroxyindolin-2-one to 2-hydroxy-1,4-benzoxazin-3-one (HBOA). 3-hydroxyindolin-2-one + O2 + reduced [NADPH--hemoprotein reductase] = 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one + H(+) + H2O + oxidized [NADPH--hemoprotein reductase] Secondary metabolite biosynthesis; 2,4-dihydroxy-1,4-benzoxazin-3-one biosynthesis; 2,4-dihydroxy-1,4-benzoxazin-3-one from indoleglycerol phosphate: step 4/5. Belongs to the cytochrome P450 family.