For example, from your terminal, if you run xxd -psd and press ctrl-f v and then enter and finally ctrl-c to exit, it outputs the following:

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$ xxd -psd
^Fv
06760a^C

What matters is the sequence 06760a^C. Let’s split it every two characters:

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06 -> ctrl-f
76 -> v
0a -> return
^C -> ctrl-c

From here, we know that 0x06 0x76 corresponds to ctrl-f v. This is the most important part. Once we have this, we can now tell Alacritty to use this escape sequence code. I use cmd + d to open a vertical pane, now we’re going to tell Alacritty to invoke the above escape sequence.

To do this, we need to add a line to key_bindings field in alacritty.yml:

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- { key: D, mods: Command, chars: "\x06\x76"  }

Now whenever you press cmd + d inside Alacritty, this will automatically be transformed into the escape sequence \x06\x76 which then opens a vertical tab in tmux.