VLAN SWITCH ACCESS PONT FORTIGATE

Hello to everyone I am an IT in my company and i need some help with our fortigate 100d.The problem is that i want all wifi users outtraffic from a specific wan in my fortigate 100d.The prolem is that I HAVE  allied telesis at-gs950/48 switch i have in port 16 my access point and i want to create a vlan i the switch and from this vlan i want my switch to connect wi th fortigate and outgoing the traffic from wan4 which is port 11.