how to edit the resignation workflow notification approve pop up

we have a requirement to edit the pop up message box asked to enter comments when an approver clicks on Approve or Reject in Resignation workflow notifications

In the below Image we can see the comment box along with that we want to add text on top of the comment box.

How can we achieve this.

I had gone through the common content/templates/workflow notification subtemplate.

But didn't find anything which is related to the pop up.

Thanks,

Dara Nagasai