Same ID for THE Multiple Items in SQL Server in ASP.NET - sql

Question

Please help as I want to insert the same ID into multiple records for the billing application.
The table structure is like below:
CREATE TABLE [dbo].[Service_Billing]
(
[Service_ID] INT IDENTITY (1, 1) NOT NULL,
[BillID] INT NULL,
[BillDate] DATETIME NOT NULL,
[Contrid] NCHAR (10) NULL,
[Description] NVARCHAR (250) NULL,
[Qty] NCHAR (10) NULL,
[Post] NVARCHAR (50) NULL,
[Period] NCHAR (10) NULL,
[Rate] NCHAR (10) NULL,
[Amount] NCHAR (10) NULL,
PRIMARY KEY CLUSTERED ([Service_ID] ASC)
);
The stored procedure looks like this:
CREATE PROCEDURE [dbo].[BillAdd]
#BillDate DATETIME,
#Contrid NCHAR(10),
#Description NVARCHAR(250),
#Qty NCHAR(10),
#Post NVARCHAR(50),
#Period NCHAR(10),
#Rate NCHAR(10),
#Amount NCHAR(10)
AS
DECLARE #ser INT = 0
SET #ser = (SELECT MAX(BillID) FROM Service_Billing) + 1
INSERT INTO Service_Billing(BillID, BillDate, ContrID, Qty, Post, Description, Period, Rate, Amount)
VALUES(#ser, #BillDate, #ContrID, #Qty, #Post, #Description, #Period, #Rate, #Amount)
Insert command is like this:
Using con As New SqlConnection(strConnString)
For i As Integer = 0 To Gridview1.Rows.Count - 1
Dim Qty As DropDownList = DirectCast(Gridview1.Rows(i).FindControl("Qty"), DropDownList)
Dim Post As DropDownList = DirectCast(Gridview1.Rows(i).FindControl("Post"), DropDownList)
Dim Period As TextBox = DirectCast(Gridview1.Rows(i).FindControl("Period"), TextBox)
Dim Rate As TextBox = DirectCast(Gridview1.Rows(i).FindControl("Rate"), TextBox)
Dim Amountttl As TextBox = DirectCast(Gridview1.Rows(i).FindControl("Amount"), TextBox)
Using cmd As New SqlCommand
cmd.Connection = con
cmd.CommandType = CommandType.StoredProcedure
cmd.CommandText = "BillAdd"
cmd.Parameters.AddWithValue("#BillDate", Date.ParseExact(billedate.Text, "dd/MM/yyyy", CultureInfo.InvariantCulture))
cmd.Parameters.AddWithValue("#ContrID", aggrmnt.SelectedValue)
cmd.Parameters.AddWithValue("#Qty", Qty.SelectedValue)
cmd.Parameters.AddWithValue("#Post", Post.SelectedItem.Text)
cmd.Parameters.AddWithValue("#Description", NewBill_Description.Text)
cmd.Parameters.AddWithValue("#Period", Period.Text)
cmd.Parameters.AddWithValue("#Rate", Rate.Text)
cmd.Parameters.AddWithValue("#Amount", Amountttl.Text)
con.Open()
cmd.ExecuteNonQuery()
con.Close()
End Using
Next
'MsgBox("Bill Processed Sucessfully", vbOKOnly)
End Using
and the output is like image below:
What I wish to do with code id like below {Updated them manually}