Main toolbar > Quick Render flyout > Quick Render (ActiveShade)

Keyboard > Shift+Q (Uses the Quick Render mode currently active on the toolbar: either Production or ActiveShade)

The Quick Render (ActiveShade) button, available from the Quick Render flyout, creates an ActiveShade rendering in a floating window.

You assign which renderer to use for ActiveShade rendering on the Assign Renderer rollout of the Render Scene dialog > Common panel.