Learn more about Config and its types here.

Learn more about Select Field Type here.
1
Add Dataslot in Config Portal
Click on the
Config Portal in Salesforce and click on Add Field button.2
Provide details
Provide a name to the dataslot and select 
Type as List Contact List Values. Click on Add Field.Click on
Edit for the dataslot and mark it as Required so that the workflow will not execute if this is not selected by the users.
3
Save
Click on
Save Field and the dataslot is now added for your users to select a list in config.To use this dataslot in the Sync workflow, you would need to change the action in Node 2 to
Get Contacts Of A particular List and provide this dataslot in Enter List ID field.Navigate here to learn how you can manage Configs for your end users.