Select a desired association. Then in the context bar below, click on Add Attribute by Start Role or Add Attribute by End Role depending on what attribute you want to add. An attribute with the correct type and multiplicity will be added to the particular class.
Adding Attributes by Association Roles
If you have two classes connected by association you may want to add attributes representing the association roles. This tutorial shows how to do it in just a click.
Adding Attributes to UML Classes by Association Roles