Using AssignMessage to set Form Parameters based on Variables

[#1]

Can someone suggest a way for me to add a form parameter to a request in an assign message policy?

Neither of these work (XML violations)

<AssignMessage name="createChallenge">
<DisplayName>createChallenge</DisplayName>
<Properties/>
<Set>
<FormParams>
<FormParam name="">{strutsTokenName}:{strutsTokenValue</FormParam>
<FormParam name=\"{strutsTokenName}\">{strutsTokenValue}</FormParam>
......