Program.cs

using Syncfusion.DocIO.DLS;
using Syncfusion.DocIO;

using (FileStream inputFileStream = new FileStream(Path.GetFullPath(@"Data/Template.docx"), FileMode.Open, FileAccess.ReadWrite))
{
    //Open the template Word document.
    using (WordDocument document = new WordDocument(inputFileStream, FormatType.Automatic))
    {
        string htmlFilePath = @"Data/File.html";
        //Check if the HTML content is valid.
        bool isvalidHTML = document.LastSection.Body.IsValidXHTML(htmlFilePath, XHTMLValidationType.None);
        if (isvalidHTML)
        {
            //Define the variable containing the text to search within the paragraph.
            string variable = "Youth mountain bike";
            //Find the first occurrence of a particular text in the document
            TextSelection textSelection = document.Find(variable, true, true);
            //Get the found text as single text range
            WTextRange textRange = textSelection.GetAsOneRange();
            // Get the paragraph containing the found text range
            WParagraph paragraph = textRange.OwnerParagraph;
            //Get the next sibling element of the current paragraph.
            TextBodyItem nextSibling = paragraph.NextSibling as TextBodyItem;
            //Get the index of the current paragraph within its parent text body.
            int sourceIndex = paragraph.OwnerTextBody.ChildEntities.IndexOf(paragraph);
            //Clear all child entities within the paragraph.
            paragraph.ChildEntities.Clear();
            //Get the list style name from the paragraph.
            string listStyleName = paragraph.ListFormat.CurrentListStyle.Name;
            //Get the current list level number.
            int listLevelNum = paragraph.ListFormat.ListLevelNumber;
            //Append HTML content from the specified file to the paragraph.
            paragraph.AppendHTML(File.ReadAllText(Path.GetFullPath(htmlFilePath)));
            //Reapply the original list style to the paragraph.
            paragraph.ListFormat.ApplyStyle(listStyleName);
            //Reapply the original list level number.
            paragraph.ListFormat.ListLevelNumber = listLevelNum;
            //Determine the index of the next sibling if it exists.
            int nextSiblingIndex = nextSibling != null ? nextSibling.OwnerTextBody.ChildEntities.IndexOf(nextSibling) : -1;
            //Apply the same list style to newly added paragraphs from the HTML content.
            for (int k = sourceIndex; k < paragraph.OwnerTextBody.Count; k++)
            {
                //Stop applying the style if the next sibling is reached.
                if (nextSiblingIndex != -1 && k == nextSiblingIndex)
                {
                    break;
                }
                Entity entity = paragraph.OwnerTextBody.ChildEntities[k];
                //Apply the list style only if the entity is a paragraph.
                if (entity is WParagraph)
                {
                    (entity as WParagraph).ListFormat.ApplyStyle(listStyleName);
                }
                else
                {
                    break;
                }
            }
        }
        using (FileStream outputFileStream = new FileStream(Path.GetFullPath(@"Output/Result.docx"), FileMode.Create, FileAccess.ReadWrite))
        {
            //Save the modified Word document to the output file stream.
            document.Save(outputFileStream, FormatType.Docx);
        }
    }
}