A test case links to a requirement
A test plan links to a requirement collection, module or module view

So if you are using collection, module or module view you can got to a requirement in the collection, module or module view, follow the link to the corresponding test case, and from there navigate to the test plan.

If that's too indirect and you really need to associate a requirement with a test plan (for example, you aren't using ar requirements collection module or module view), consider linking the requirement to the first test case in the plan. You could make test case something that's informative rather than a real test case.

Alternatively paste the link to the test plan in the requirement artifact. You won't be able to do traceability analysis or report on the link (or follow it from the test plan back to the requirement), but you will be able to navigate from the requirement to test plan.