As it's name suggests a front add-on attaches to the objective (front) lens of the scope rather than the ocular (rear) lens of the scope
A front add-on consists of an objective lens which focusses IR onto a sensitive camera and a small screen which displays what the camera sees.
when the add-on is fitted and you look through the scope in the normal way. what you are actually looking at is the screen at the rear of the add-on.
Since the screen produces visible light, scope lens coatings are not a problem and the add-on optics are designed such that no scope focus control is needed.
Here's a link to the Pulsar F455 NV front add-on
Digital NV Front Attachment F455 | Forward Series | Pulsar
You might think that sticking something onto the front of a scope which you then look at through the scope would cause a loss of zero - and in earlier front add-on this was a problem, but the current generation of front add-ons seem to have cracked this issue.
When you buy a front add-on, you need to know outside diameter of your objective lens so that you get the correct adaptor to allow the add-on to be a snug fit onto your scope.
Hope that helps
Cheers
Bruce