You can use Save()
method like this:
string itemId = // id from AddFromTemplate() xmlNode
string fieldId = "{some-id-of-the-field}"; // id of the field
XDocument fieldXml = new XDocument(
new XElement("sitecore",
new XElement("field",
new XAttribute("itemid", itemId),
new XAttribute("language", "en"),
new XAttribute("version", 1),
new XAttribute("fieldid", fieldId),
new XElement("value", "new value of the field")
)
)
);
XElement fieldResponse = scClient.Save(fieldXml, "web", credentials);
EDIT:
In response to your comment: No, you cannot use field name instead of field id. Still you can use GetItemFields
method exposed in VisualSitecoreServiceSoapClient
to list all the fields of the item and then determine what is the id of the field with given name:
public XmlDocument GetItemFields(string id, string language, string version, bool allFields, string databaseName, Credentials credentials)