How can i add to a modal the small triangle like in speech bubble

I mean like this
you need to pass event through the function in order for the popover to know where to be placed, and that should add the lil triangle.
How can i add to a modal the small triangle like in speech bubble

I mean like this
you need to pass event through the function in order for the popover to know where to be placed, and that should add the lil triangle.