Tmux popups are actually floating windows that you can drag around the screen. They always open in the middle (by default) when you open them, no matter where you leave them.
Here are a couple of keybindings I use to open up popup windows.
bind C-g display-popup -E "ta ~/git" bind -n M-g display-popup -E "tmux new-session -A -s scratch"
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Also check out this long form post for more about how I use tmux.