Step 1: Follow guide on https://support.google.com/analytics/ans...3666?hl=en to create new User-ID View on GA.
Step 2: Login your Xenforo Admin -> Appearance -> Templates -> find template name: google_analytics
Find
Code:
<xf:if is="$xf.options.googleAnalyticsAnonymize"> 'anonymize_ip': true,
</xf:if>
Code:
<xf:if is="{$xf.visitor.user_id}"> 'user_id' : '{$xf.visitor.username}',
</xf:if>
Done tutorial, you can check result via view-source.
Reference