Verify token from Blockstack

I am building an application using Meteor, and i need to implement Blockstack authentication. I am almost done, but the last thing to do is to verify the token from this service. I am using verifyAuthResponse. As the first argument, i am passing the authResponseToken, but i have no idea what is nameLookupURL is and where i can get it. I will be very grateful for any help. Maybe i am doing it in the wrong way?

The nameLookupURL is the url of a blockstack node, e.g. https://core.blockstack.org/v1/names/
In blockstack.js, core.blockstack.org is the fallback node if no url is provided.

What are you building?