Error while converting feild to Link button to CRM Form 2013 online

function ConvertToLink(fldName) {

    var btn = "<a href='javascript: void(0);' onclick=\"\'\', \'windowname1\', \'width=600, height=650\');  return false;\" style='color:blue;text-decoration:underline !important'>Test Account</a>";

    var ctrl = Xrm.Page.ui.controls.get(fldName)._control;


    // Add the new button 

    //ctrl.get_element().innerHTML += btn;

    // Hide the textbox 

    ctrl.get_element() = 'none';


when i alert the “ctrl” i get Error “undefined”.

I think the problem is at .controls.get(fldName)._control;

Please help!