If your question was not about the way to construct an intercept leg, but about the way to get a runway centerline drawn on the nav display:
Enter a runway into the route. The approach choice screen should show not only the available instrument approaches but, at the end of the list, also the available runways (sec). If you choose a runway without instrument approach, the box should automatically draw an extended centerline to a default distance out, or ask you to specify a runway extension distance.