The splitw command supports specifying a target.
Use -t to specify a pane.
Ctrl-B :splitw -t 0
On a very useful side note:
will help to identify what pane to use.
Want to do that frequently to the same pane (like pane number zero, instead of the "current pane")? Then create a keyboard binding.
Expansion to answer:
The following will create the layout that was requested by the ASCII art in the question.
(1): Start with a brand new window. Maybe not strictly required, but I did this to simplify testing.
(2): Make a vertical split (Ctrl-B :splitw -h )
(3): Optional, but recommended: do something in pane #1 so that the pane looks different, just so you can see that this works as intended.
(4): press Ctrl-B
(5): Although you need a colon, that is part of the following text. Paste in the following text.
:splitw -t 0 ; splitw -t 0 -h ; selectp -t 1 ; swap-pane -s 3 ; selectp -t 1 ; kill-pane -t 1
(6): Press Enter, to run the compound command.
Note (7): The approach shown depends on specific pane numbers, so step (1) is extremely recommended. I'm not saying that this solution is highly adaptable for other layouts, and so hard-coding the keyboard binding may produce unexpected results in other starting situations. However, this does show how to do what was specifically requested, so this may be a useful demonstration on how to do this manually (which is often helpful when designing an automated approach to a task).
Note (8): I'm quite wary about this approach, especially with the use of kill-pane. It seems that could easily lead to lost work if something doesn't work quite as expected. If you're gutsy enough to want to try putting this into a key binding, consider using the confirm statement, as shown with the default binding for Ctrl-B x (shown with Ctrl-B ?).