If you need to schedule a single-year, one-off income, select a single event (green dot) without selecting an end event (red dot).
Do not over-think the semantics of the "Annual" option. A one-off income will occur only once. In most cases a one-off income would be entered in an annual amount. Select Annual to indicate that the amount entered is an annual one.
Which screen you choose to enter this income will depend on how the income should be taxed. If the income counts as earnings, which are taxable and subject to NI, enter it on the Employment screen. If the income is non-taxable (or you are entering it net of taxes) or is taxable but not subject to NI, enter it on either the Other Income or Windfalls screen with Taxation set setting accordingly.