Hi @yuni -
DelineateIt is currently made for creating watersheds that flow to particular points on a stream, for example drinking water facilities, hydropower plants or towns. So we often have existing spatial data showing the point location of those places, and these points are the outlets for DelineateIt. If you do not have existing points, you can create them manually. For example, you could create a new point vector layer (like a shapefile) and use a basemap to visually locate the location of a dam, and add that location as a new point in your vector layer. Then that point vector layer would be the outlet input to DelineateIt.
Hopefully that helps, but if not, please describe in more detail what you need to do and I can offer other suggestions.