To add a goal to the chart, you will want to start in the chart editor (click the dropdown arrow to the top right of a chart).
From here, you will want to select the "Options" tab. Navigate to the bottom of the "Options" tab and will you see a "Goals" section. Click "+Add a Goal" as shown below:
After you select "+ Add a Goal", enter the goal value in the "Goal" field and the name you would like your goal displayed as in the "Label" section.
Make any changes to the color, click "Add", and the changes will be reflected as shown below:
Click "Save" in the top right corner, and your Goal will be good to go!