how can i add JavaScript code after once user key authenticated with the domain name

I wanna create sale analytics and performance measurement crawler or tool. Basically, I am a Full stack developer and just playing with something and having difficulties where i should start and what to do. I wanna create a application where people will register with their website domain name. Noow i will provide the script with a key... something like this

window.omnipredict_key = 'xxxxx-xxxx-xxxxxx-xxxxxx-xxxxxx';

I need to authenticate the key then my script will be included into that particular website and i am done and plan next to develop my script.

The idea if about to crawl users activities.

Anyway, can some one atleast suggest me where i neeed to go and what i have to do ? how can i make this possible? Make it like CND anyone can copy and [past with there keys and use my script. If the keys wrong i will put a message in console.

Can anyone help me with it please

thanks Regards Qaisar