To add only a specific goal to the widget, you need to create a custom widget, set the Segment to any specific segment you need, meaning “all users”, “organic traffic” etc. After that, select the dimension, which is usually “months of year”, and then select a specific goal ID, which you can find in your Google Analytics.
After that, just find that Goal ID in the Metrics drop-down menu, and voilà, you're all set.