The best option is to draft a thank you note (like below), send it to your team leader, and ask them to please forward it to your anonymous donor(s).
Dear Anonymous Donor,
Thank you so much for your gift toward my mission trip! Although I do not know who you are, I thank God for you and for your generosity. I excited to see what God does on this trip, and excited that you are a part of it with me! - Name