Need to add link in the description of start form variables in O365 Nintex Workflow

I'm not sure I understand exactly what you're asking so I will guess that you want the URL of the current item stored in a variable.

If so then use the Build String action and assign {Workflow Context: Current item URL} to a text variable

Actually we need to add the link in the workflow start form which means that the link has to be shown even before the workflow execution starts for taking user's inputs. Only way to control those settings in O365 is through the gear icon which appears for the variables opted for showing in the start form(as fas as we found).