Check whatsapp online status

According to this answer for iPhone developers on WhatsApp FAQ;

WhatsApp provides two ways for your iPhone app to interact with WhatsApp:

1-)Through a custom URL scheme

2-)Through the iOS Document Interaction API

If your application would like to open a WhatsApp Chat with a specific contact, you can use our custom URL scheme to do so. Opening whatsapp:// followed by one of the following parameters, will open WhatsApp and perform a custom action.

NSURL *whatsappURL = [NSURL URLWithString:@"whatsapp://send?text=Hello%2C%20World!"];
if ([[UIApplication sharedApplication] canOpenURL: whatsappURL]) {
    [[UIApplication sharedApplication] openURL: whatsappURL];
}

On another answer for Android developers on WhatsApp FAQ, you can send a message via WhatsApp with an ACTION_SEND intent. You can also bypass the system picker, you can set sendIntent.setPackage("com.whatsapp"). For custom URL scheme:

WhatsApp provides a custom URL scheme to interact with WhatsApp:

If you have a website and want to open a WhatsApp chat with a pre-filled message, you can use our custom URL scheme to do so. Opening whatsapp://send?text= followed by the text to send, will open WhatsApp, allow the user to choose a contact, and pre-fill the input field with the specified text.

Here is an example of how to write this on your website:

<a href="whatsapp://send?text=Hello%20World!">Hello, world!</a>

There are two parameters at the time which are according to answer given for iPhone developers, which are app and send. Go to first link to find detailed information.

Also check WhatsAPI library by venomous0x.