In general, it must be done at the airport (some phone agents can do so, but in practice will not).

You can call the 1P number to ask about what your standby chances look like on the earlier flight to judge whether or not it would be worthwhile to show up at the airport to attempt the standby. Mr. Chicken may offer you the chance to standby when you check in, and a human can certainly place you on standby either at check-in or at the gate or RCC.

I have yet to utilize the $25 confirmed standby so cannot offer advice therein.