tx.cs

Appendix appendix = new Appendix()
{
    Field1 = "Field 1 Text"
};

Data data = new Data() {
    Field = "Field Text",
    Appendix = appendix
};

using (MailMerge mailMerge1 = new MailMerge())
{
    mailMerge1.TextComponent = textControl1;
    mailMerge1.RemoveEmptyBlocks =
        removeEmptyBlocksToolStripMenuItem.Checked;
    mailMerge1.MergeObject(data);
}