Yes, you can set a custom URL for donors to get redirected to after they've donated!
Log into your Donorbox account and go to your dashboard.
Click on the Edit link, next to your Campaign name. Go to the "Amounts" tab.
Tick the box that reads "Redirect to a custom URL after donation". A text box will appear for you to add the URL that you want donors to be taken to after they've donated - as shown below.
Once you add your URL, hit "Save" and you are all set. Please ensure the URL you add is a valid link, or else donors may think that your donation form is faulty.