ewsRequest.js

var req3 = "<?xml version='1.0' encoding='utf-8'?> \
      <soap:Envelope xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/' \
                     xmlns:t='http://schemas.microsoft.com/exchange/services/2006/types'> \
       <soap:Header> \
           <t:RequestServerVersion Version='Exchange2013'/> \
        </soap:Header> \
        <soap:Body> \
        <GetItem xmlns='http://schemas.microsoft.com/exchange/services/2006/messages'> \
          <ItemShape> \
            <t:BaseShape>Default</t:BaseShape> \
            <t:BodyType>Text</t:BodyType> \
            <t:AdditionalProperties> \
             <t:ExtendedFieldURI PropertyTag='0x007D' PropertyType='String' /> \
            </t:AdditionalProperties> \
          </ItemShape> \
          <ItemIds><t:ItemId Id='" + Office.context.mailbox.convertToEwsId(Office.context.mailbox.item.itemId, Office.MailboxEnums.RestVersion.v2_0) + "'/></ItemIds> \
        </GetItem> \
         </soap:Body> \
      </soap:Envelope>";

Office.context.mailbox.makeEwsRequestAsync(req3, async (res) => {
    if (res.status === 'succeeded') {
        console.log(res);
    }
});