I’m not sure what you mean by “focus a specific window”. (Win)+n may be what you want; it will go to the nth task on the taskbar. If you want a certain combination (e.g., +5) to always go to a certain program, you should pin it to the taskbar. (If the task has multiple windows, hold while you press n repeatedly, and it will cycle through the windows.)
Another answer is to invoke Windows Task Manager (e.g., by Ctrl+Shift+Esc), go to the “Applications” tab (if you aren’t there already), select the task you want to go to (click or cursor down or type the first letter of its name), and press Enter or click on “Switch To” or double-click on the task name.