Manufacturers have different requirements.
Option 1 adds the board outline to the top copper layer.
Option 2 shows the steps to produce a separate board outline plot.
Select the “Top Copper” Plot in the left hand pane, click the “Layers” tab and toggle the ‘N’ to ‘Y’.
The board outline will now be included in the top copper plot.
Click the “Add Plot” button and then select “Gerber” from the options.
A new Plot is listed in the left hand pane, select this and from the “Output” tab rename the plot to “Board Outline”, then click the “Layers” tab.
Toggle the ‘N’ to ‘Y’ and the new plot is enabled.
Click ‘Run’ to produce all the selected Gerber plots.