Vendor - Add vendor Id info as User ID of Contact

Hi Expert,

i have the following requirement :

New button called u2018Add Dummy contactu2019will be available via toolbar, and when user click on this button, dummy contact date will be filled in automatically.

The mandatory data that should be filled in are the following:

First Name = Dummy

Last Name = Dummy

Email = noreplay@ xxxx.com

User ID: vendor number must appear automatically eg VEN-000078

I have created the scripting u2013 Toolbar for Contact object, and First Name, Last Name and Email are filled in automatically.

How I can fill in the vendor number like VEND-0078 in User ID field ?

Could you send me the scripting code for this?

Thanks & Regards,

Anacia