Project Insight does not calculate backwards from a fixed end date like Microsoft Project. However, you can set up your projects and templates to determine a project start date from your fixed deadline date. 

Once the project tasks have been set up with the work that must be accomplished to complete the project, you can use the total duration calculation of the project to determine a start date. Make note of the total duration of the project and add a one-day duration task in your finalized project task list (normally as the last task on the list) with a "Must Start On" constraint of the final due date.

Create a "Finish-to-Start" predecessor to the first task with a lead time equal to the total duration you noted earlier. This will move the initial task to the date calculated from the end date and continue calculating all of your other tasks forward. Once the date of the initial task is calculated, it is best go in and modify the Project start date accordingly. 

You can further monitor changes to the project which will impact the final end date by adding an auto-alert which will email you if the project scheduled end date changes. Even though that task you set for the due date will not change with changes to the schedule, the schedule end date will change any time any task pushes past the previously scheduled end date. You will immediately know that you need to make changes in order to maintain your originally scheduled end date.

Helpful Resources

Firm end date video

Intelligent scheduling video