Create directive for ionic component

I want to create the directive for ion-input. For example, this directive append a ‘http://’ prefix to url string, if it’s less. However I don’t understand how get value;
My code:

import {Directive, ElementRef} from '@angular/core';

  selector: '[urlFormat]',
  host: {
    '(blur)': 'onBlur()',
export class UrlFormatDirective {
  constructor(private el: ElementRef) {


  private onBlur(): void {
    console.log('BLUR', this.nativeElement.value); // undefined


<ion-input type="url" urlFormat></ion-input>