You can't do what you are trying easily - setting the focus to a particular control is a pain in MVVM generally.
It also goes against what MVVM is all about - the VM shouldn't really know focus is going to be set at all.
If you do need to do it, you can use attached properties - see
here[
^]
But, depending on your application, it may be simpler to set the focus in code behind on some event or other.