There are two methods of doing this, one where the part is added to the route (shown here), and the other where the route is added to the part

Navigate to Data⇒Production⇒Route

Find the route that you want to add the part to (using search if required), and then double click.

Select the Associated Parts Tab and then click the Add Part button

In the pop up screen, select the part number that you want to add to the route and enter the Material Spec (If a material is added when a part is created, it will automatically appear here but can be overwritten), Production Lead time and Minimum value/Minimum order charge as required and then click Save

The part now shows in the list of parts associated with that route