It is not possible to hide the html attribute completely, but you can look into obfuscating it. This often relies on Javascript and will only make it harder to get the password, but it will be there and the script used for obfuscating will be there too as it will neet to be parsed by the users browser.
What you want to do probably need an external sollution to enter the password automatic.
It would require a different tool to create the vouchers in a database or directory (AD) where the tool (script) creates the same password and the user only need to enter the username.
This could be done with a powershell script talking with AD in a windows environment and using IAS for RADIUS authentication in stead of using the internal Database. This way your admins would run the script, enter the username and it is done.
If you dont know how to create and manage this kind of system, or you dont have the environment required. It should not be to difficult for your administrators to remember to put the same password each time they create a new user (voucher), and it probably takes 3 seconds of extra time. Since this is a voucher system, it is probably not an unmanagable amount of administration?