You can share a project risk by simply using the Article feature. For example, "Have not received final approval on design". If you are comfortable you could also Mention the person you are waiting on. The benefit of using the Article feature is that you can go back into the risk and add, for example, a Mitigation for the risk once you have identified one.
For more information about Articles check out: http://neudesic.zendesk.com/entries/20440748-articles