Using Parameters for a Simple Data List Lookup on a Listbuilder

I've got a listbuilder related to an Event, which is the context for the page. For a few columns, I'd like to include a lookup, but need to pass the context (e.g., Event Preferences) so it only returns the relevant entries for the event. This all seems fairly straightforward, using parameters. However, no matter what I try, it's passing the literal value.

Is this a bug? Am I missing something? Thanks!

Mitch

<OutputField Name="PREFERENCES" Caption="Event preferences" DataType="String">
<LookupInfo>
<SimpleDataList SimpleDataListID="115817C4-7184-4587-AC1B-CB7DFF88296C">
<c:Params>
<c:Param ID="EVENTID">
<c:Value>FORM!CONTEXTID</c:Value>
</c:Param>
</c:Params>
</SimpleDataList>
</LookupInfo>
<MultiValue DataType="String" Delimiter=" | " UseForFiltering="true" />
</OutputField>