Method 1: You can right click on any plant or object in the content explorer and select add to favorites
Method 2: You can right click on a plant or object on your background image.
Both methods work the same. You should get a popup box where you can place them under the favorites folder or create your own folders by using the Add Folders button.

This will be the same favorites list that shows up in Planner so you won't have to add them again.