The command your are looking for is
split-window [-dhvP] [-c start-directory] [-l size | -p percentage] [-t target-pane] [shell-command]
Create a new pane by splitting target-pane: -h does a horizontal split and -v a vertical
split; if neither is specified, -v is assumed. The -l and -p options specify the size of the
new pane in lines (for vertical split) or in cells (for horizontal split), or as a percentage,
respectively. All other options have the same meaning as for the new-window command.
You could use it from your current window by entering command mode, Ctrlb,: and entering at the prompt:
splitw -v -p 50 'your command'
This would split your current window vertically in half (50 percent) with your command executed in it.
If you use this command frequently, you can always bind it to a key sequence, eg.,
bind '~' split-window -v -p 50 'htop'