install-mu-module composer command missing

I want to move the necessary plugin file to the must-use plugins directory using composer, because the web user has no privilege to write to that directory.

According to readme file I should be able to do this:

"scripts": {
"install-wp-mysql-mu-module": [
"@composer --working-dir=wordpress/wp-content/plugins/index-wp-mysql-for-speed install-mu-module"
],
"post-install-cmd": [
"@install-wp-mysql-mu-module"
],
"post-update-cmd": [
"@install-wp-mysql-mu-module"
]
},
But I get the error “Command “install-mu-module” is not defined.” and no such script seems to exist in the plugin directory.
I have adjusted the working dir to my own environment, that is not the problem.