I don't think you can directly do it, you can either send the user to a contact or a group selection screen as said by @Nitin Gohel. For sending a text to a particular contact you would probably do this,
NSURL *whatsappURL = [NSURL URLWithString:@"whatsapp://send?text=Hello%2C%20World!&abid=143rnjk4545352523"];
if ([[UIApplication sharedApplication] canOpenURL: whatsappURL]) {
[[UIApplication sharedApplication] openURL: whatsappURL];
}
abid is the address book id of the contact you would like to send a text, but for a group you don't have any such thing that is visible from the outside of whatsapp's sandbox.
So I guess, it is not possible to do it.
Just a wild guess about sending a message to multiple contacts,
P.S: I don't know whether this would work or there is not official documentation to support this. Try abid's with commas.
NSURL *whatsappURL = [NSURL URLWithString:@"whatsapp://send?text=Hello%2C%20World!&abid=143rnjk4545352523,32545365436d34,3f4fret454f,f4w454etgegdt"];
if ([[UIApplication sharedApplication] canOpenURL: whatsappURL]) {
[[UIApplication sharedApplication] openURL: whatsappURL];
}
Also I came across this, which I think says us this is not quite possible with the existing whatsapp url scheme support.