Google lets you send IMs via voice command using a select group of third party providers2
To get this feature, you need to have the latest version of the Google app downloaded on your mobile device, along with the latest version of your messaging app. The Google app is available from the Google Play Store. You can install the latest version by clicking on the sourcelink.
Once you have everything set up, sending an Instant Message is as easy as saying "Okay Google, send a WhatsApp message to John" followed by your dictation. And your message needs to be in English because that is the only language that this method supports at the moment. Google says that it is working to add other languages in the future.
source: Google (Android) via AndroidAuthority