By default there is an input bar at the top of your lists that lets you quickly enter a task you want to do today.
This bar can be removed completely or changed so that tasks and projects entered in there (p: project name) get added to the Master List without being scheduled to today.
To change this, go to the settings on the top right.
Click on the "Layout tab" and scroll down to 'Select main input bar". Click this to open the settings.