Please add the code snippet below to the file "bp-custom.php" in the path "wp-content/plugins" :
https://gist.github.com/KaineLabs/80c7fe3111a59474bdcfd6c0c27adfdd
Ps: if you didn't find the file "bp-custom.php", just create a new one.
This code snippet will add the code to the "Thank you for your registration" page.