You can request to be put on the standby list online 6 hours before your first flight leaves.

For example: For MCI-MSP-SEA, if requesting standby for MSP-SEA you can do it online 6 hours before your MCI-MSP departure. You will need to get to the gate of your standby flight to see if you got it or not.

BTW, First Class will fill up quickly with elites and coach might be oversold. Do not rely on seat maps!

Check fare availability to see how busy the flight is.