Originally Posted by fone

Yes, you can pre-assign seats, if only you are in a premium cabin.

For SQ, you can only choose seats online (on SQ's website) if you booked the itinerary on SQ's website. Otherwise, you need to wait till check in (online check in opens 48 hours before the flight). They will not allow you to preselect seats by calling if you are in Y, maybe unless you pay for "prefered seating" which is the emergency exit rows.

For passengers in R or F or J, you can preselect the seats by calling SQ. When you call SQ, do ask them for the SQ booking reference too, just in case you need it later on. They will be able to find you with just your flight number, flight date and name.

So if you are in Y and did not booked SQ's website, then as warreng24 mentioned, you can only tell them your preference and select your seat at the 48 hour online check in mark.