How to add "separator" in the swimlanes by milestone?

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I have a few milestones in my "timescale" that I'd like to have separators go down into the swimlanes. I've searched online but can't find how to do it in my project timeline, so I thought I'd ask here.

The "red circle" is the separator from the timescale, but I'd like to be able to add milestones, and have the separators into the swimlanes. Is this possible? If so, how?

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