mailHandler.js

 item.body.setSelectedDataAsync(
            `<div class="<div class="customerClass"><span style="color:white;display:none" class="CustomerClass">hidden Text</span></div>`,
            {coercionType: Office.CoercionType.Html},
            function (asyncResult) {
                if (asyncResult.status ==  Office.AsyncResultStatus.Failed) {
                    console.error('failed!!!');
                } else {
                    console.log('success');
                }
            });