I am trying to configure Service Cloud chat with our website. To correlate the chat session with a user in our own internal system I would like to pass our own user ID to the contact object in Salesforce. 

Here's the code I have so far: 

liveagent.addCustomDetail("user id", 123);

liveagent.findOrCreate('Case')
    .saveToTranscript('CaseId')
    .showOnCreate();

liveagent.findOrCreate('Contact')
    .map('user id', 'user id, true, true, true)
    .saveToTranscript('ContactId')
    .linkToEntity('Case','ContactId')
    .showOnCreate();

I am certain the field "user id" exists in our salesforce object.

Any help would be much appreciated!