A visit tag is a specially configured measure, which you can use to mark a certain visit. When the threshold of the measure triggers, the measure value is assigned to the visit.
Review the below link for all the information required to configure the visit tag.
You will have to create a custom sensor rule to capture the userid/username. Once you will have this information in the argument/return value then you can use as a visit tag.
Review the below link for more understanding about the sensor configuration.