. . . . how to get the "ComputerID" to get a new License key for Vegas 15 Pro . . . .
The ComputerID is generated when you elect to register the software offline, normally you would register/activate the software online using the serial number that was emailed to you when you purchased the software.
If you have previously installed and registered the software and lost the original serial number, then you should be able to recover it by:
using any web browser go to magix.com
select the My Account icon
select My Products
There you will see your registered program(s), serial number(s) etc.