Adding Rebrandly

Hello,

I am wanting/ needing to connect my Rebrandly account to my site so I can create custom links for a site not on SquareSpace but want them branded as my domain through Rebrandly.   i.e.  the URL for the dentition would be ABC but my site is 123; with Rebrandly I can create a link such as 123.com/me and it will take me to ABC.com/home.

I believe I need to add Java script code to the 404 page and then create a folder for verification however, I am unsure how to go about this, 

I have attached the two articles from Rebrandly with the information. 

Rebrandly article "Connect a Domain via Rebrandly Plugins / Configure your Alias Domain"

Rebrandly article "Alias domain verification"

Any and all help is much appreciated! 

Thank you all!