The documentation says that you can add the --compose option in git send-email to "Call a text editor to edit the introductory message for patch series."
If you want to automate this action and generate text using a script. You can set the environment variable $GIT_EDITOR to a script. It will get the temporary file name for the text in the command line argument. The contents of this file will be inserted into the message after the script exits.
The command for git send-email will look like this:
$GIT_EDITOR="/path/to/your/script" git send-email ...
And your script might look like this:
#!/bin/bash echo "Your message" > $1
source share