I found the following solution, first you'll need the whatsapp id:
Matching with reports from some other threads here and in other forums the login name I found was some sort of: international area code without the 0's or + in the beginning + phone number without the first 0 + @s.whatsapp.net
For example if you live in the Netherlands and having the phone number 0612325032 it would be [email protected] -> +31 for the Netherlands without the 0's or + and the phone number without the 0.
public void sendWhatsAppMessageTo(String whatsappid) {
Cursor c = getSherlockActivity().getContentResolver().query(ContactsContract.Data.CONTENT_URI,
new String[] { ContactsContract.Contacts.Data._ID }, ContactsContract.Data.DATA1 + "=?",
new String[] { whatsappid }, null);
c.moveToFirst();
Intent whatsapp = new Intent(Intent.ACTION_VIEW, Uri.parse("content://com.android.contacts/data/" + c.getString(0)));
c.close();
if (whatsapp != null) {
startActivity(whatsapp);
} else {
Toast.makeText(this, "WhatsApp not Installed", Toast.LENGTH_SHORT)
.show();
//download for example after dialog
Uri uri = Uri.parse("market://details?id=com.whatsapp");
Intent goToMarket = new Intent(Intent.ACTION_VIEW, uri);
}
}