Create a Script Based Calculated Value and associate it with your custom attribute.  A sample script is below.  Note that workItem.getValue(WorkItemAttributes.TYPE) returns the id of the workitem type so you will need to map these to the workitem type name within your script as I have done for defect and task. 

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dojo.provide("com.test");
dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes");
(function() {
var WorkItemAttributes= com.ibm.team.workitem.api.common.WorkItemAttributes;
    dojo.declare("com.test", null, {

        getValue: function(attribute, workItem, configuration) {

var type = workItem.getValue(WorkItemAttributes.TYPE);
if (type == "defect") {
  type = "Defect";
} else if (type =="task") {
  type = "Task";
}
var id = workItem.getValue(WorkItemAttributes.ID);
var state = workItem.getValue(WorkItemAttributes.STATE);
if (!state) return ""; //returns an empty string prior to workitem being saved
return "PRNT: "  + type + id;

        }
    });
})();