Hello Susmitha,
You have to add the rule and follow these steps,
1. Rules -> ‘Add’ - > ‘Add Action’.
2. Select ‘Set a field value’ option in first dropdown in Action.
3. Select your field with help of ‘Select a Field or Group’ option in ‘Field’.
4. Select your external data source list value in ‘Value’.
This rule you can apply in OnLoad or whenever you will get external data source values.