Either click Create New or the Expand icon in the lower right corner of the group. Then New > Custom or New > Flag and Move. You can't choose a custom flag - it would either be tomorrow or this week (which is the last working day of the week as set in Options, or Friday for most people)
Or you can just click in the Flag field - set the quick click to the desired preset. To set the quick click, right click on the flag then choose Set quick click. ( The flag won't be visible until you move over it - unless it is flagged.)