Questions Combobox New Field

This is my first attempt to add a field to a form.  I'm trying to add a field "Modified by" to the sales order form, financial settings tab.  Ideally, I'd like this to contain the same select options as the owner field, but I could not seem to get it to work out, no matter how much I gleaned off other fields.  Right now I'd settle for a drop-down with various user names available.  I am successful at creating a customization project, creating a field and having it added to the screen as a text box.  However, I can't seem to get it to act as a drop-down.  I have the control set to ComboBox and have the following entered as the attributes:

[PXDBString(64) ]
[PXStringList(new string[] { "mh", "jl", "js", "mf", "ds", "mk", "tq", "mt" },
     new string[] { "mhuston", "jlong", "jsorstok", "mfarrell", "dschmidt", "mkane", "tquiney", "mtrumble" })]
[PXUIField(DisplayName="Modified By")]

Is there something obvious that I'm missing?  I'm working in 2019R1.  Any insight is appreciated!