def ThisFoldername = JOB_NAME.replace("/","/job/");

def theString = "<a href='https://jenkins.com/job/" + ThisFoldername + "/" + BUILD_NUMBER + "/execution/node/5/ws/'>Workspace</a>";

manager.addShortText(theString, "black", "white", "0px", "white");

or added any href link string will not be html link it will be just a full string displayed 

 Note it works file for 

manager.createSummary("green.gif").appendText(theString , false, false, false, "black");

Please Fix.

 Thanks