Dynamically update checkbox variable's label - classic UI and portal

var sMsg="A checkbox message"; if (typeof spModal != 'undefined') { //--loaded from Service Portal g_form.setLabelOf('attachment_attached', sMsg); } else { variableSysID = 'ad87f965db6f1c1011eaa0214b9619c4'; //--the actual sys id of the variable itself var checkboxvar = document.getElementById('ni.IO:' + variableSysID + '_label'); checkboxvar.innerHTML = sMsg;  

} 

from the HTML (inspect element):

<div id="variable_IO:8da14aa1dbaf1c1011eaa0214b96190c" class="form-group sc-row "><fieldset> <input type="HIDDEN" name="sys_original.IO:8da14aa1dbaf1c1011eaa0214b96190c" id="sys_original.IO:8da14aa1dbaf1c1011eaa0214b96190c" container_id="4da14aa1dbaf1c1011eaa0214b96190c" class="cat_item_option"><div class="io_table col-xs-12 form-field input_controls " style="padding-left:0px"> <input type="HIDDEN" name="IO:8da14aa1dbaf1c1011eaa0214b96190c" id="IO:8da14aa1dbaf1c1011eaa0214b96190c" container_id="4da14aa1dbaf1c1011eaa0214b96190c" class="cat_item_option"><div class="row sc-row sc_checkbox is-required" parent_container_id="8da14aa1dbaf1c1011eaa0214b96190c"> <div class="col-xs-12 catalog-checkbox"> <span class="input-group-checkbox" style="margin-left:15px"> <input id="ni.IO:ad87f965db6f1c1011eaa0214b9619c4" name="ni.IO:ad87f965db6f1c1011eaa0214b9619c4" aria-labelledby="ni.IO:ad87f965db6f1c1011eaa0214b9619c4_label" class="checkbox" type="checkbox"> <span id="status.IO:ad87f965db6f1c1011eaa0214b9619c4" title="" mandatory="true" oclass="changed required-marker label_description" class="mandatory required-marker label_description" data-original-title="Required - must be checked before Submit">&nbsp;</span> <img id="section508.IO:ad87f965db6f1c1011eaa0214b9619c4" src="images/s.gifx" alt="Required - must be checked before Submit" title="Required - must be checked before Submit" style="width:0px; margin-right:0px; margin-left:0px; padding-left:0px; padding-right:0px;" data-original-title="Required - must be checked before Submit"> <label id="ni.IO:ad87f965db6f1c1011eaa0214b9619c4_label" for="ni.IO:ad87f965db6f1c1011eaa0214b9619c4" class="checkbox-label" oncontextmenu="return contextQuestionLabel(event, 'ad87f965db6f1c1011eaa0214b9619c4', '7'); " aria-expanded="false">Authorised flexible working application form attached to the request?</label> </span></div><input id="IO:ad87f965db6f1c1011eaa0214b9619c4" name="IO:ad87f965db6f1c1011eaa0214b9619c4" type="HIDDEN" class="cat_item_option" value="false" gsftcontainer="IO:8da14aa1dbaf1c1011eaa0214b96190c" aria-required="true" data-type="checkbox"><input id="sys_original.IO:ad87f965db6f1c1011eaa0214b9619c4" name="sys_original.IO:ad87f965db6f1c1011eaa0214b9619c4" type="HIDDEN" class="cat_item_option" value="false"></div></div></fieldset></div>