Sending whatsapp messages to a number using the API

I am trying to send a message to a WhatsApp number from my app built using Angular 4 and Ionic 3 . I am trying this on a button click event. This is my code.

HTML

<div padding>
<button id="addBtn" (click)="WhatsApp()" ion-button color="primary" block>WhatsApp</button>
</div>


TS

WhatsApp(){
return new Promise(resolve => {
console.log("Before API")
this.http.get("https://api.whatsapp.com/send?phone=121234567891&text=I'm%20interested%20in%20your%20car%20for%20sale").map(response => response.json()).subscribe(data => {
resolve(data);
console.log("After API",data)
}, err => {
});
});
}


I want to send a message on the button click. So where must I specify the number from which and number to which I am going to send and is the way I have done is correct? Thanks in advance

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