Each URL copied directly from the event page contains an "instance" of that page.  This is a piece of the URL that is not needed to direct potential donors to the correct page.
You can remove that piece of the URL to shorten and make more user friendly.

For example, if the URL is copied directly from searching for the participant it looks like this:

The "instance" that can be removed is: This portion is found after the event ID, which reads in this example as ievent=469293 and ends before the "&" symbol right before the supporter ID, which reads as supID=3243300021.

The end result URL looks like: