You can use "Start Marker" and "End Marker" to specify the portion of the song to be exported as a WAV file.

Click the "Start Marker" or "End Marker" button on the transport tool bar to display the markers on the rulers of both the Track Editor and the Musical Editor.
If you drag these markers to the start and end positions for export, only the specified section of the song is exported as a WAV file.

This can also be done by clicking and holding the mouse's left button on the section with the green border (the top area on the ruler).
After the mouse pointer changes into a hand cursor, drag-and-drop to set the start and end positions.