Hi Ben,
Typically, legend symbols are associated with layers on the map to aid in interpreting the symbols displayed on the map canvas, rather than within the popups themselves. Popups are generally configured to show attribute information, related records, and media such as images or charts.
If you are looking to enhance popups beyond these default capabilities, you might need to utilize the ArcGIS API for JavaScript for more advanced customization options. I recommend exploring the ArcGIS Developers site for customizing popups with the ArcGIS API for JavaScript.PopupTemplate | API Reference | ArcGIS Maps SDK for JavaScript 4.29 | ArcGIS Developers
Additionally, the video below from an Esri event provides a walkthrough of different popup customizations. I suggest you start by skipping to minute 19:18 to see if it addresses what you are looking for; if not, you may find other useful options throughout the presentation.ArcGIS API for Javascript: Better UX with Popups (youtube.com)
I hope this helps. Thank you!