Ditto what Bwolf said.
Just talk to an agent at ticketing, or at your gate, depending on your needs:
"
Assistance in the Airport and with Boarding
Upon arrival at the airport, please inform a Southwest Airlines Agent or Skycap at your first point of contact, either at the Skycap podium or the ticket counter, if you need an airport wheelchair and/or assistance within the airport.
Priority preboarding is available for those who have a specific seating need to accommodate their disability and/or need assistance in boarding the aircraft or stowing an assistive device. If a Customer has a disability and requires preboarding, the Customer should request a Preboarding Document from our Customer Service Agent at the departure gate. The Preboarding Document serves as notification to our Operations (Boarding) Agent that the Customer has a need to preboard. It's important to keep in mind that Customers who preboard may not occupy an exit seat. Customers with disabilities who have any other special needs related to boarding should speak with our Customer Service Agent at the departure gate."