you can also adapt this one liner
command -range TmuxSession execute "!tmux.session " . @*
or if you do need multiline visual selection change @* to substitute(@*,"\n"," ","g")
The @* register stores current selection (for linux, on windows there could be different naming convention).
The -range is only to avoid an error if you want to execute this command with range marks, i.e. '<,'>TmuxSession.