As the readme for Source Han Sans mentions, neither of these characters is encoded in Unicode yet, so writing them isn't supported by most input methods. (Wikipedia uses SVGs to show the characters; the font recognizes the ideographic description sequence.)

However, once support arrives, it will be as easy as typing "biang", since there's currently no character with that pronunciation. It could be confused with a two-syllable word "bi ang", but there's no such word in Mandarin.