Heh, I have something similar, with a tmux session-start script[1] that's the first thing run by my default iTerm windows. It displays and lets me select from existing sessions via prompt tab completion. Or I can just enter a new session name at the prompt.
In iTerm, this would be a start command of:
login -fp YOUR_USERNAME /Path/To/local/bin/mux --prompt
Or used directly from a normal command line (with zsh completion [2]) as:
mux
Apologies to tmuxinator users for conflicting with 'mux'; obviously feel free to rename it. This is really just a part of my personal setup vs. a formally published utility, and I don't use tmuxinator.
[1] https://github.com/jwhitley/tilde-local/blob/master/local/bi...
[2] https://github.com/jwhitley/zshrc/blob/master/.zfunctions/_m...