Use SetDefaultColumnValues to set default column values.

// Full implementation an vary and can be much longer and more verbose,
// this example shows the base MetaData defaults for forms and also
// provides little error checking.

var formsFolder = list.RootFolder.Folders.FirstOrDefault(x => x.Name == "Forms");
if (formsFolder != null)
{
    var xmlSb = new StringBuilder();
    XmlWriterSettings xmlSettings = new XmlWriterSettings
    {
        OmitXmlDeclaration = true,
        NewLineHandling = NewLineHandling.None,
        Indent = false
    };

    using (var xmlWriter = XmlWriter.Create(xmlSb, xmlSettings))
    {
        xMetadataDefaults.Save(xmlWriter);
    }

    var objFileInfo = new FileCreationInformation
    {
        Url = "client_LocationBasedDefaults.html",
        ContentStream = new MemoryStream(Encoding.UTF8.GetBytes(xmlSb.ToString())),
        Overwrite = true
    };

    formsFolder.Files.Add(objFileInfo);
    clientContext.ExecuteQueryRetry();
}

list.SetDefaultColumnValues(list, columnValues);