Dear …
On behalf of __(name of organization)__ I would like to extend our warmest gratitude to you for speaking at the launch of our __(name fundraiser)__ fundraising event. Your first-hand experience with __(type of cause)__ drove home to each of us, just how very important this __(type of support for cause)__ really is. Your enthusiasm was very infectious and you did a commendable job.
I know I also speak for others when I say that you left us all highly motivated to make this fundraiser the best it could possibly be. Once again thank you, __(name)__, for your noteworthy contribution. It was vital to the success of our __(name fundraiser)__ fundraiser.
Sincerely,