Go to Contacts > Contact Lists and select the list. Then click the pencil icon to edit and add the script in the Call Script box.
This will appear in the Phone Script tab in the power dialer:
Tip: Use dynamic Terms!
You can also insert dynamic terms into your script so that you can autofill personal information about that contact. In the example below, we simply used the contact first name dynamic field so that my contact's first name will appear directly in the phone script:
Once you save your phone script, you will now see the contact's first name directly in the script! You can do this for all sorts of fields, including custom fields.