Thanks both of you for the feedback,
Again I am new to Azure, I am running PHP 7.3 stack.
I am guessing Windows is the OS? I have not picked any OS' during the set up following this
https://learn.microsoft.com/en-us/azure/app-service/app-service-web-tutorial-php-mysql,
But I have been using PowerShell.
As a somewhat work around, I moved the composer.bat and .phar files from the siteExtenstions/ComposerExtentions/Commands to my site/wwwroot and then was able to run composer in cmd
So it seems the problem is that it wont install composer globally?