Please review What Are Custom Fields? documentation before getting started.
Creating Custom Fields via Mobile App
From the Home menu, tap Settings to enter the Settings Menu.
From the Settings menu tap Custom Fields to view a list of all previously created Custom Fields.
From the Custom Fields management list, tap the Add button to create a new Custom Field.
Enter a unique Name for your new Custom Field, and select which Data Type you would like to store under it. Then tap Save.
Your new Custom Field will now be available to associate with Product Definitions in your account.