The following video shows how to setup boundary conditions and loads in Mechanical.
Note: The applied pressure should be negative to correctly simulate trachea behavior. This is not stated clearly in the video, but it is entered correctly.
There have been some issues in selecting the faces to apply the pressure load to. If you are finding that you are not able to select the inside of all the faces and it selects the outside of some or all of them, go through the steps in the following video. (If you think you are having issues, but aren't sure, you can go through the video too and it will give you the correct setup as well.)