var doc = new RadFixedDocument();

var editor = new RadFixedDocumentEditor(doc);
editor.InsertRun("Sample Document");

Table table = new Table();
for (int i = 0; i < 100; i++)
{
    TableRow row = table.Rows.AddTableRow();
    for (int j = 0; j < 5; j++)
    {
        var cell = row.Cells.AddTableCell();
        cell.Blocks.AddBlock().InsertText("Test");
    }
}
TableRow lastRow = table.Rows.AddTableRow();
var lastCell = lastRow.Cells.AddTableCell();
lastCell.Blocks.AddBlock().InsertText("##FieldHolder");

editor.InsertTable(table);

var provider = new PdfFormatProvider();
var docBytes = provider.Export(doc);

var document = provider.Import(docBytes);

SimplePosition holderPosition = new SimplePosition();
RadFixedPage holderPage = null;

foreach (var page in document.Pages)
{
    foreach (var element in page.Content)
    {
        if (element is TextFragment fragment)
        {
            if (fragment.Text == "##FieldHolder")
            {
                var fragmentPosition = fragment.Position;
                holderPosition.Translate(fragmentPosition.Matrix.OffsetX, fragmentPosition.Matrix.OffsetY);
                holderPage = page;
                fragment.Text = "";
            }
        }
    }
}

TextBoxField textField = new TextBoxField("SampleTextBox")
{
    MaxLengthOfInputCharacters = 500,
    IsMultiline = true,
    IsPassword = false,
    IsFileSelect = false,
    ShouldSpellCheck = true,
    AllowScroll = true,
    Value = "Sample content",
};

VariableContentWidget widget = textField.Widgets.AddWidget();
widget.Rect = new Rect(new Point(holderPosition.Matrix.OffsetX, holderPosition.Matrix.OffsetY), new Size(250, 50));
widget.RecalculateContent();

document.AcroForm.FormFields.Add(textField);
holderPage.Annotations.Add(widget);

File.WriteAllBytes(@"..\..\result.pdf", provider.Export(document));