Custom Page Context

There are a few different ways you could go about this.

The first way is that you can definitely include /board-id/${coreNode.id} in the URL and then use the FreeMarker below to retrieve it on the custom page. (I do this in a few of my custom pages.)

<#assign boardId = webuisupport.path.rawParameters.name.get('board-id', '') />

A similar way would be to use a query string, such as ?boardId=${coreNode.id} in your URL, which would use the FreeMarker below to retrieve.

<#assign boardId = http.request.parameters.name.get('boardId','') />

The third way (if you didn't want to pass something in the URL for some reason) would be to use a session attribute, as shown below.

<#-- Set the session attribute on the originating page -->
${http.session.setAttribute('boardId', coreNode.id)}

<#-- Retrieve the session attribute value on the custom page -->
<#assign boardId = http.session.attributes.name.get('boardId', '') />

(The empty strings in all of the examples above represent the default/fallback value.)

Hope this helps!