Hello Siobhan,
You did not detail what WiNG 5 device, but if using a wireless controller (e.g. NX, VX, RFS) or access point that supports this feature, you will use the following via the CLI:
enable [enter]
config [enter]
captive-portal xxxxxx [enter] (x represent captive portal policy. You can use tab key to auto-fill the name. If typing, capitalization matters with names)
webpage-location advanced [enter]
webpage external welcome xxxxxxxx [enter] (x represents complete URL)
webpage internal welcome use-external-success-url [enter]
com wr [enter]