There is a recipe in which Se powder is dissolved in hot sodium sulphite solution, and then ammonium chloride is added. It is apparently a dangerous procedure because Se is poisonous. Look for Dassonville T-55 formula.
I have read that it's very difficult to get all of the Se to dissolve. The diluted toner is said to not keep well, but other reports say that if diluted in hypo, it lasts well. I haven't tried it.