Which channel technology? This would seem a strange requirement from the provider for VoIP, a called numbers are normally sent in the request URI.
I’d imagine you would need to dial it as though the complete phone number was *67, and you then sent the remainder as (early media) DTMF. I thought there was a GUI way of doing this, but a quick search only comes up custom dialplan:
In any case, I think you need to ask the provider exactly what they require over the wire.
If this is analogue, you should be able to use the standard DTMF sending pause codes.