With the help of DiegoM I was able to install Mongodb's Web SDK into my project.
But now a new problem has arisen. My plan is to build a mobile app, exporting to Cordova. But then obviously redirection from the Realms Web SDK built in OAuth2 Facebook Login will not work anymore. Also I'm not sure if the built in functions of the Web SDK will work inside Cordova. They might though, as Cordova is just a browser app, if I'm not wrong.
My problem now is to let the user sign in to Mongodb Realm using the SDKs built in Facebook OAuth2 without redirection.
(The next problem will be using API calls or the built in SDK to retreive and post Database information as the Realm documentation, although pretty detailed, lacks very much of detailed code examples. There are just confusing variations of incomplete code snippets for the same thing scattered across multiple sites. But that will be the next step and maybe part of a future question when I get the sign in with Cordova working. :D)