How to move the cursor position using mvvm

Hi,

in my form having 3 textboxes. by default i am displaying the cursor at first textbox. i have set tab index for all 3 textboxes. if the first text box is empty then the cursor automatically moves 3 rd textbox. how to do this ? i set the tab index is in order. do not change the position of textboxes.

i am developing this application in mvvm and nothing to write the code in .cs file.

Thanks,
Venkatesh.P