Fogbugz API :: Searching on a custom field

My version of Fogbugz is v8 and I'm trying to use the API to refine results by a custom field that i've created, so far i've got t开发者_运维问答he following:

/api.asp?token=[token]&cmd=search&q=project:"=116"+plugin_customfields_at_fogcreek_com_environmentf52:LAMP&cols=sTitle,sPersonAssignedTo,ixBug,environment,product

But the API keeps giving me an error saying

The requested search axis 'plugin_customfields_at_fogcreek_com_environmentf52' is not recognized.

Does anybody know what I am doing wrong?

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/api.asp?token=[token]&cmd=search&q=project:"=116"&plugin_customfields_at_fogcreek_com_environmentf52:LAMP&cols=sTitle,sPersonAssignedTo,ixBug,environment,product

You use & instead of + to indicate different search criteria.