UpdateItem

POST https://outlook.office365.com/EWS/Exchange.asmx HTTP/1.1
Authorization: Bearer xxxxxx (omitted)
Content-Type: text/xml; charset=utf-8
Host: outlook.office365.com
Content-Length: 1262

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
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>
    <UpdateItem MessageDisposition="SaveOnly" ConflictResolution="AutoResolve"
    xmlns="http://schemas.microsoft.com/exchange/services/2006/messages">
      <ItemChanges>
        <t:ItemChange>
          <t:ItemId Id = "AAMkAGI3NDEzZjRhLWU3ZjktNGViNy04MTI1LWFhOWRiZDRlY2QwNABGAAAAAADBaXFYA4KPQqsQBpmZF2+2BwAa6KJzOcvaRKcc5UfLbF5tAAAA4fK1AAACk4HLLuxASqDPNieBkIv8AANIKlICAAA=" ChangeKey="CQAAABYAAAACk4HLLuxASqDPNieBkIv8AANIb3mG"/>
          <t:Updates>

            <t:SetItemField>
              <t:FieldURI FieldURI = "item:Categories" />
              <t:Message>
                <t:Categories>
                  <t:String>Workout</t:String>
                </t:Categories>
              </t:Message>
            </t:SetItemField>
          </t:Updates>
        </t:ItemChange>
      </ItemChanges>
    </UpdateItem>
  </soap:Body>
</soap:Envelope>