Instead of using the string approach written in the sitecore documentation https://doc.sitecore.com/developers/93/sitecore-experience-manager/en/the-restful-api-for-the-itemservice.html when sending the data for the item creation, for example:

    xhr.send("{\n \"ItemName\": \"Home\", \n \"TemplateID\": \"76036f5e-cbce-46d1-af0a-4143f9b557aa\",
    \n    \"Title\": \"Sitecore\" \n}");

 

you can use this approach instead to set the sortorder during Create Item:  

      var obj = {};
      obj['ItemName'] = "Home";
      obj['TemplateID'] = "76036f5e-cbce-46d1-af0a-4143f9b557aa";
      obj['__Sortorder'] = "you custom sort order value";
      xhr.send(JSON.stringify(obj, null, 4));