Receipt emails go out automatically after a donation is made. To add a custom receipt message to your donation campaigns first click Donation Campaigns on the left side of your Dashboard.
Then click Manage on the campaign you would like to add a custom receipt message to.
Next, click Settings and scroll down to the Custom Receipt Message section. Type your message in the text box and click Save. You can also send yourself a test.
Below is an example of a test receipt. You will notice the custom message in the pink box.