To send the linebreak, try using %0a
e.g.
strMessage = LblCompany.Text & "%0a" & "Invoice Number : " & txtInvoiceno.Text & "%0a" & "Invoice Date : " & DtInvdate.Value.ToString("MM/dd/yyyy") & ""
web.Navigate("whatsapp://send?phone=" & txtcontact.Text & "&text=" & strMessage & "")