I have heard about URL-schemes but I cannot return back to my app after Whatsapp opens.
I referred this link http://www.whatsapp.com/faq/en/iphone/23559013
Here is code snippet which I copied into my app:
NSString * msg = @"YOUR MSG";
NSString * urlWhats = [NSString stringWithFormat:@"whatsapp://send?text=%@",msg];
NSURL * whatsappURL = [NSURL URLWithString:[urlWhats stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]];
if ([[UIApplication sharedApplication] canOpenURL: whatsappURL]) {
[[UIApplication sharedApplication] openURL: whatsappURL];
} else {
UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"WhatsApp installed." message:@"Your device has WhatsApp installed." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
[alert show];
}