Have I to insert php in the command field and -q -f /your/path/to/awbs/tools/invoice_cron.php in the imput to command field?No. "Input to command" is something to be piped into the command...your command is the whole thing. e.g.:
Command: php -q -f /your/path/to/awbs/tools/master_cron.php
And the input field should be empty, as there is nothing to pipe into the command. The input field is usually used for email text or some other bunch of text to inject into a particular command.
See the Webmin docs:
http://doxfer.com/Webmin/ScheduledCommands