ContentDefinitionManager.AlterTypeDefinition("AssociationMember", a => a
    .WithPart("CommonPart")
    .WithPart("MyContentType")
    .Creatable());

ContentDefinitionManager.AlterPartDefinition("MyContentType", cft => cft
    .WithField("Website", a => a.OfType("LinkField").WithDisplayName("Website")
        .WithSetting("FieldIndexing.Included", "True"))
    .Attachable());

var myItem = _orchardServices.ContentManager.New("MyContentType");
_orchardServices.ContentManager.Create(myItem);
var websitePart = myItem.Parts.FirstOrDefault(x => x.Fields.Any(y => y.Name == "Website"));
var websiteLinkField = websitePart .Fields.FirstOrDefault(x => x.Name == "Website") as LinkField;
websiteLinkField.Value = "http://www.google.com";
websiteLinkField.Text = "Link to google";
_orchardServices.ContentManager.Publish(myItem);