Gabriel,
If you are in the ArcGIS Administrator and want to change from Single Use license to Concurrent Use license, you must change the software option to one of the Concurrent Use licensing. In addition, you must specify the license manager hostname to retrieve the Concurrent Use license. This is the hostname or IP address of the machine where the ArcGIS License Manager was installed. If its on the same machine, provide the hostname of your machine. If you're not sure where that machine is, you may need to contact your administrator.
I'm not sure if this is what you mean by "host". If not, can you provide a little more detail, where you're at in the software and what workflow you're following?