1. Create an empty Landing Page in Mautic.
...and name it "Redirect".
(In fact, we even tend to name it "Redirect LP (do not delete, do not enable)" - just to make sure.)
2. Set this page to "unpublished".
Yes, really! Or else the following won't work.
3. In the Landing Page properties, set a 302 Redirect to the desired external URL.
4. In System Configuration, set "Mautic's root URL" to the Landing Page that you just created.
That's all :)