Hi @GeorgeJonesRIGOV,
Part 1: Create an app with a drop down menu for editing.
Yes, this is possible in ArcGIS Online using forms. Form builder allows you set up your own custom data entry form that can contain different input types such as a combo box, or drop-down menu.
You can create Forms using Field maps designer as you mentioned, and you can also create them in Map Viewer. The form can then be used in Map Viewer or other desktop apps you create based on the web map, such as Instant Apps or Experience Builder. Search for Instant Apps templates that include the ability to edit, like the Sidebar App.
For more information on creating custom forms, check out this blog article with step-by-step instructions, options, and best practices.
Link to Blog
Part 2: add on to the list of options in the drop down menu.
The options in the drop down menu are populated using an attribute list, aka list, or domain. You can add more options to the list on the Data tab of the Layer’s Item Details page. Here is a link to the ArcGIS Online help to get you started.
Link to documentation
