Insert record into another table based on Id in SQL Server

Hi i have this query i use to select table Id, but the query is using Username to select the Id, please how do i correct that. What i want to achieve is this, i want to use the Id of a table to insert record into another table

code

  protected void GetMergedAll_ItemCommand(object source, DataListCommandEventArgs e)
    {
        try
        {
            if (this.Page.User.Identity.IsAuthenticated)
        {
                string username = this.Page.User.Identity.Name;
                string id = e.CommandArgument.ToString();
        
           // string username6;
            if (e.CommandName == "Insert")
            {
                //int userPostId = e.CommandArgument;
                TextBox txtComments = (TextBox)e.Item.FindControl("txtComments");
              //  TextBox txtaddress = (TextBox)e.Item.FindControl("txtsaddress");
               // TextBox txtrollno = (TextBox)e.Item.FindControl("txtsrollno");
                SqlCommand cmd = new SqlCommand("InsertComment", con);
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.AddWithValue("@MyComments", txtComments.Text);
               //cmd.Parameters.AddWithValue("@UserName", username);
               // cmd.Parameters.AddWithValue("@FriendUserName", username);
             //   cmd.Parameters.AddWithValue("@Id", id);
                con.Open();
                cmd.ExecuteNonQuery();
                con.Close();
                cmd.Dispose();
                Response.Write("<script type=\"text/javascript\">alert('Comment Posted Successfully!!!');</script>");
               // BindDatalist(username);
            }
        }
        }
        catch (Exception ex)
        {
            throw ex;

        }
    }

procedure

ALTER PROCEDURE [dbo].[InsertComment](
	@MyComments VARCHAR(MAX)
	
)
AS
BEGIN
	DECLARE @Id INT;
	SELECT @Id = u.Id FROM USERPost AS u WHERE u.Id = @Id
	INSERT INTO UserCommentPost (postId,MyComments )
	VALUES ( @Id, @MyComments)
END

Any help please