FieldName

public string FieldName { get; }

public void InsertCheckBox()
{
    Document doc = new Document();
    DocumentBuilder builder = new DocumentBuilder(doc);

    // Use MERGEFIELDs with "TableStart"/"TableEnd" tags to define a mail merge region
    // which belongs to a data source named "StudentCourse" and has a MERGEFIELD which accepts data from a column named "CourseName".
    builder.StartTable();
    builder.InsertCell();
    builder.InsertField(" MERGEFIELD  TableStart:StudentCourse ");
    builder.InsertCell();
    builder.InsertField(" MERGEFIELD  CourseName ");
    builder.InsertCell();
    builder.InsertField(" MERGEFIELD  TableEnd:StudentCourse ");
    builder.EndTable();

    doc.MailMerge.FieldMergingCallback = new HandleMergeFieldInsertCheckBox();

    DataTable dataTable = GetStudentCourseDataTable();

    doc.MailMerge.ExecuteWithRegions(dataTable);
    doc.Save(ArtifactsDir + "MailMergeEvent.InsertCheckBox.docx");
}

/// <summary>
/// Upon encountering a MERGEFIELD with a specific name, inserts a check box form field instead of merge data text.
/// </summary>
private class HandleMergeFieldInsertCheckBox : IFieldMergingCallback
{
    /// <summary>
    /// Called when a mail merge merges data into a MERGEFIELD.
    /// </summary>
    void IFieldMergingCallback.FieldMerging(FieldMergingArgs args)
    {
        if (args.DocumentFieldName == "CourseName")
        {
            Assert.AreEqual("StudentCourse", args.TableName);

            DocumentBuilder builder = new DocumentBuilder(args.Document);
            builder.MoveToMergeField(args.FieldName);
            builder.InsertCheckBox(args.DocumentFieldName + mCheckBoxCount, false, 0);

            string fieldValue = args.FieldValue.ToString();

            // In this case, for every record index 'n', the corresponding field value is "Course n".
            Assert.AreEqual(char.GetNumericValue(fieldValue[7]), args.RecordIndex);

            builder.Write(fieldValue);
            mCheckBoxCount++;
        }
    }

    void IFieldMergingCallback.ImageFieldMerging(ImageFieldMergingArgs args)
    {
        // Do nothing.
    }

    private int mCheckBoxCount;
}

/// <summary>
/// Creates a mail merge data source.
/// </summary>
private static DataTable GetStudentCourseDataTable()
{
    DataTable dataTable = new DataTable("StudentCourse");
    dataTable.Columns.Add("CourseName");
    for (int i = 0; i < 10; i++)
    {
        DataRow datarow = dataTable.NewRow();
        dataTable.Rows.Add(datarow);
        datarow[0] = "Course " + i;
    }

    return dataTable;
}