Type a syllable with Telex, for example viet → việt.
A list of Nôm characters appears below the text field: tap one, or press 1–0.
With ↑↓ you move through the list, Esc closes it.
The PA button next to Convert PA shows the word before the cursor as PA, the phonetic script (what is PA?); a click puts it in the text. With Phonetic annotation on, a syllable you do not pick a Nôm character for stays in Quốc Ngữ until you press Space: then it turns into PA. Enter picks the first Nôm character. The address writer.html?pa=1 has it on from the start.