There are two ways I can of think to do this: Give the window a name (via a x:Name attribute on the Window tag, and then construct a binding like this (assumes the name of the window is ‘ThisWindow’):
<Button Command="CommandGetsCalled" CommandParameter="{Binding ElementName=ThisWindow}" />
For something more general (doesn’t rely on giving the current Window a name), the binding can be constructed like this:
<Button Command="CommandGetsCalled" CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}" />