The feature you’re looking for is called “First Play”.
Here’s how the DVDA online help describes it.
Note: There are some symbols displayed that can’t be shown here so you’ll have to look for yourself.
Set the DVD introduction (first play) media
Use the Insert Introduction Media option to choose a media file that you want to use as your DVD start item.
Select an object in the Project Overview window.
Click the down arrow next to the Insert Object button and choose Introduction Media .
The Introduction Media is displayed in the Project Overview window.