While Supplier can be added when creating a Transfer, it's usually best to make them beforehand.
Creating a Supplier
First navigate to the Inventory section, this is where all your inventory management will be done. Then underneath In/Out, click Suppliers and then the green plus sign in the top right corner. Fill out as much of the information as you have available, noting that the red asterisk are required.
Depending on the State, you may be required to put in their License Number too.