I have for years credited SQ J/F flights to PPS with miles to UA. Surprisingly my number is now a KF number even though I am no longer PPS Club! (I never signed up for KF)

I would avoid setting it up online, as some of my travelling partners did. Doing it online sets you up with a Krisflyer account, which eventually becomes a PPS account once you've done the travelling.

Go to an SQ sales office (preferably in SIN) and do it there. Tell them "miles to UA, PPS qualification points only."

If this gets confusing, just give them your UA number, and then get retroactive credit to PPS.