Events that have Official Charities associated with their event will have a donations tab on the top menu. If there is no donation tab, fundraising is not available for participants.
If donations are available you can only raise funds for the charity that is associated with the event.
If there is another charity that you were interested in raising funds for you can contact the race director to see if they are interested in listing your charity of choice. Selecting the Contact Us field on the Race Info page.