Qlik Sense comes with an amazing feature that allows you to automatically associate your tables based on the field names and data in those fields. To automatically associate your data based on Qlik's recommendation, click on the magic stick on the right of the screen,  right above the + sign. The engine will automatically create the associations: 

You can see the details of the associations by clicking in the middle circle between the bubbles. Qlik will automatically rename the associated fields so it can then join the tables through this common field name. You can change this name from within the Association name text-box as shown in the preceding screenshot.

Click on Customer association should you wish to use a different associating field instead of using the one recommended by Qlik Sense.