Don’t know why you were downvoted. In any case, all terminals can be configured to start with a specific command and arguments. So, depending on your terminal, you might need to read the documentation, and/or search the web.

In alacritty config, this is:

shell:
  program: <CMD>
  args:
    - <ARGS>

Then one of these:

1. <CMD> is the path to tmux, and you have configured tmux to run the shell of your choice. Search the web for how.
2. <CMD> is the path to your shell, and it supports launching in tmux. Search the web for how.

For me, it’s the second one. I use fish, and I launch it with fish --command=tmux. So the above config looks like this:

shell:
  program: /usr/bin/fish
  args:
    - --command=tmux