When creating a New Reservation (see How do I create a new Reservation?), rather than selecting an available time slot, select the Enter Manually option

Select a date and time from the scrolling calendar and tap Next, then tap Confirm

Please note: this option disregards system calculated availability and allows double-booking

If the time/date you have selected causes a double booking, the following warning message will display:

Tapping Confirm following this message, will result in a double booking: