"xxx12581;" is the secondary phonetic under the "L" key, which has "xxx12580;" as the primary phonetic. You just need to double-hit the "L" key quickly to bring up "xxx12581;".
Another "difficulty" in finding it is that "xxx12581;", in BoPoMoFo input sequence, is never stand alone or the first phonetic of any combination. You need to enter anther phonetic first, e.g. "xxx12583;" under the "Y" key, then "xxx12581;" becomes selectable... quite tricky, isn't it.