Using the Xfinity App to activate

Hi,

I am going to attempt a replacement of my modem on Monday.  I assume somewhere in the process

I will be asked for a Network name and password.  Do I enter the default Network and password that is on the back of the Gateway,

or do I name my own Network and my own personal password at that time?  Also, I have a CMAC address and a HFC Mac address.

Which one do I use?   Thanks in advance...