Metadata Create SObject With Auto-Number

Sample SOAP Request Message - Creating an SObject with Auto-Number

<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
 <soapenv:Header>
  <ns1:SessionHeader soapenv:mustUnderstand="0" xmlns:ns1="http://soap.sforce.com/2006/04/metadata">
   <ns1:sessionId>3e8700Dx00000007S6Q!8dgyjntyVDawsOhg.bJSmk10x20ZEqeoOyWLwToLuWPEl8bc1_n4ebHT5l.B38SCfE6g_kux35VdI4_srtghhmCbvI3hzdmXW16eo7BD1wL.W5QyTACW</ns1:sessionId>
  </ns1:SessionHeader>
 </soapenv:Header>
 <soapenv:Body>
  <create xmlns="http://soap.sforce.com/2006/04/metadata">
   <metadata xsi:type="ns2:CustomObject" xmlns:ns2="http://soap.sforce.com/2006/04/metadata">
    <fullName>sample__c</fullName>
    <deploymentStatus>Deployed</deploymentStatus>
    <description>created by the Metadata API</description>
    <enableActivities>true</enableActivities>
    <label>sample Object</label>
    <nameField>
     <displayFormat>AN-{0000}</displayFormat>
     <label>sample__c Name</label>
     <type>AutoNumber</type>
    </nameField>
    <pluralLabel>sample Objects</pluralLabel>
    <sharingModel>ReadWrite</sharingModel>
   </metadata>
  </create>
 </soapenv:Body>
</soapenv:Envelope>

Sample SOAP Response

<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns="http://soap.sforce.com/2006/04/metadata">
  <soapenv:Body>
    <createResponse>
      <result>
        <done>false</done>
        <id>04sx00000000Do4AAE</id>
        <secondsToWait>2</secondsToWait>
        <state>InProgress</state>
      </result>
    </createResponse>
  </soapenv:Body>
</soapenv:Envelope>