how to create a key of web api for use in another website

To the best of my knowledge the key isn't any special value as such. So you can give any value. To make it easier to generate unique (and cryptic looking) values using GUID is the best approach.

 

The important aspect is the WebAPI consumer shouldn't share the key with anybody.

 

As far as the provider is concerned all calls made from an identical key will be billed together.

 

You may wish to wait to see what others forum members have to say.