Kathy,
This blog post should walk you through the steps on how to do this.
Thank you! That was helpful. In the linked article you posted, I really liked the planes showing flight direction, very cool. I just found something too that would also work. Your posted link's article (#5) made me think of the symbology gallery in ArcGIS Pro, for a possibility. So I checked there and here's a picture of the gallery's line with arrow symbol:
Original blog link is 404. I'm guessing it was this one though: Rotating point symbols in web maps (esri.com)