To create an External Source model in Spotlight Web:
In Spotlight web, navigate to the Dynamic Planning > Model.
Click the + Circle icon from the Grid or List layouts to create a new model.

Click the tabular icon to indicate that you want to create an External Source Model.

Give the new model a name and an optional description.

Click the Save button.
Proceed with adding fields. See Creating New ESM Fields Using Model Manager.
Click the Save button again when done.