you can also adapt this one liner
command -range TmuxSession let @a='!echo tmux.session ' . substitute(@*,"\n"," ","g") | @a
or if you do not need multiline visual selection
command -range TmuxSession let @a='!echo tmux.session ' . @* | @a
The @* register stores current selection. So you can join selected text to any other register with your custom command (a in this example, note that you will lost this register text) and then execute it.
The -range is only to avoid an error if you want to execute this command with range marks, i.e. '<,'>TmuxSession.
@* is current selection for linux, on windows there could be different naming convention.