The documentation for this is good if not a little difficult to find.
Facebook Graph API - Page Tokens
After initializing node's fbgraph, you can run:
var facebookAccountID = yourAccountIdHere
graph
.setOptions(options)
.get(facebookAccountId + "/accounts", function(err, res) {
console.log(res);
});
and receive a JSON response with the token you want to grab, located at:
res.data[0].access_token