Answer
For an initcontext parameter specifically, use a semicolon before initcontext rather than a question mark, following this pattern: .../web/page/PageClient/PageName;initcontext=ParamName:'value'. This is a different URL mechanism from OData-based page filtering (which does use a question mark for its own $filter query string) — initcontext is Aurena's own page-initialization-parameter mechanism and uses the semicolon convention instead, which is easy to mix up with the more commonly-referenced $filter syntax.