Hi,
You'll need to a DHCP helper address. This can be done on your IAP or on another network device such as a switch (depending on your network archetecture).
Here's the commands on the IAP CLI:
(Instant Access Point)(config)# ip dhcp <profile-name>
(Instant Access Point)(DHCP Profile <profile-name>)# server-type <centralized>
(Instant Access Point)(DHCP Profile <profile-name>)# server-vlan <vlan-ID>
(Instant Access Point)(DHCP Profile <profile-name>)# dhcp-relay
(Instant Access Point)(DHCP Profile <profile-name>)# dhcp-server <DHCP-relay-server>
(Instant Access Point)(DHCP Profile <profile-name>)# vlan-ip <DHCP IP address> mask <VLAN mask>
(Instant Access Point)(DHCP Profile <profile-name>)# end
Here's more details about the settings and the GUI config.