Impact Designer provides you with the ability to set a success URL for your forms. This means that you are able to create your own success page and re-direct donors to it after they've completed submitting their gift.
The advantage is that it helps continue the interaction - you can share more information about your organization and your programs, or introduce other ways they can get involved like becoming a volunteer or attending an event.
The first step would be for you to create your own custom success page with the message you want your supporters to see. Once you've set up your success page, you can then go to the form and add the URL of your success page.
From the form, go to Settings at the top menu and click on Success URL.
Enter the URL for your custom success page and click on Apply Changes.
Next, click on Save from the top menu.
To publish your changes, go to Form at the top menu and click on Publish Form.
A confirmation message will display. Click on Publish Form to continue.
You can choose to have multiple forms all re-direct to the same success URL so long as the information you have on your custom success page is relevant to the donor regardless of which form the donor is coming from.
If you choose not to specify a success URL, your form will use the standard Impact Designer success page.