This solved my problem, listening for GotFocus for the XamComboEditor and then setting IsDropDownOpen to True.

uiElement.GotFocus += (e, args) =>
{
   ((XamComboEditor)uiElement).IsDropDownOpen = true;
};

Setting IsEditable does not make this work. Setting this to false makes it so I can type into the combobox to begin filtering which is not what I want. I want to be able to type in the combo box. What I wanted to add it initially when the user clicks on the combobox before typing, have the list open up. That is what the readonly one will do.