Involved in indole glucosinolate biosynthesis. Catalyzes methoxylation reactions of the glucosinolate indole ring. Converts the hydroxy intermediates 4-hydroxy-indol-3-yl-methylglucosinolate (4OH-I3M) and 1-hydroxy-indol-3-yl-methylglucosinolate (1OH-I3M) to 4-methoxy-indol-3-yl-methylglucosinolate (4MO-I3M) and 1-methoxy-indol-3-yl-methylglucosinolate(1MO-I3M), respectively