I tried to look on the forum but didn't find a solution. How can I accomplish this?At the event there's e.X and e.Y of the cursor position, but I haven't found on the UltraTextEditor a way to get the Index at which the drop happen so I can setup SelectionStart and insert the necessary data.
If the UltraTextEditor is in edit mode, there will be a child TextBox control in it. So you could set AlwaysInEditMode to true and then use 'this.UltraTextEditor.Controls[0]' to get the TextBox control.
Once you have the TextBox, you can use the GetCharIndexFromPosition method.
https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.textbox.getcharacterindexfrompoint?view=netframework-4.7.2
The e.X and e.Y params in a drag event will be in screen coords, of course, so you will likely have to use PointToClient to convert the point first.