API keys is mandatory ?

I want to build a string for downloading a static map and send it via
SMS to a cellular phone, so who receive the SMS can directly click on
the link and open the map.
I have my API key associated with my domain, but I See the link works
fine also without the key ...
Anybody can make clear to me this point ? Can I avoid to add the API
key at the end of the string (so the SMS is shorter that 160
characters) ?
If is mandatory to add the key, is it correct to add the key I got for
my domain ? (I don't understand because it has no relation with the
user that will click and open the map from its internet connection).
Thanks
Marco

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