You can visit the view settings in the forecast perspective and choose to show flagged tasks. All flagged tasks will be shown in the forecast perspective.
Or you can designate a forecast tag in the forecast perspective. Any task with that tag will show up there.
Personally, I have a tag called Today, which indicates any task that can be completed today. I configured the Forecast Perspective as @wilsonng suggested:
If I can’t do something before a certain date, it will not bother me, but
The day it becomes actionable, it will be in my Forecast perspective until I actually do it.
I have a custom action, attached to a keyboard shortcut, to quickly toggle the Today tag, and
I also created a custom perspective which identifies everything which is available, not tagged with Today, and not due soon (so, basically, everything that does not appear in the Forecast). I use this for things which fall in between what I should do as soon as possible, and the “Someday / Maybe” category.