Check the Wiki for tutorials please, they include movable XML TopLevelControls too.
https://wiki.esoui.com/Main_Page
Example:
https://wiki.esoui.com/SimpleNotebookTutorial/part6
Your UI needs a control which got movable="true" and mouseEnabled="true".
Can be set via lua functions <control>:SetMouseEnabled(true) and <control>:SetMovable(true) too.
Add an event handler for OnMoveStop to save the actual <control>:GetLeft() and <control>:GetTop() to your SavedVariables and upon loading of your addon once reanchor the TopLevelControl to GuiRoot TOPLEFT TOPLEFT, and set it's x and y position to the saved left and top values of your SavedVariables.