I started using Houdini 14 for a month now and had a lot of trouble finding the option to add custom viewport markers. And after spending the whole , I finally found the option hidden in “Visualizers” in Display options.
I have attached the image with this post. I hope this is useful for everyone.
There's a menu of visualizers in the toolbar down the right side of the viewport (the icon looks like a map marker). You can add/edit viz'rs there and turn them on or off.