autofill.js

 document.getElementById('postcode').addEventListener('keyup', function(e) {
   if (!e.key.match(/[0-9]/)) return;

   const postcode = e.target.value;
   if (postcode.length !== 7) return;

   const url = `https://postcode.teraren.com/postcodes/${postcode}.json`;

   fetch(url)
     .then(response => response.json())
     .then(json => {
       document.getElementById('prefecture').value = json.prefecture;
       document.getElementById('city').value = json.city;
       document.getElementById('suburb').value = json.suburb;
     })
     .catch(error => {
       console.error(error);
       ['prefecture', 'city', 'suburb'].forEach(id => {
         document.getElementById(id).value = '';
       });
     });
 });

postcode.teraren.com.html

<form>
  <input type="text" id="postcode" placeholder="1600023" maxlength="7" autofocus>
  <input type="text" id="prefecture" placeholder="東京都" disabled>
  <input type="text" id="city" placeholder="新宿区" disabled>
  <input type="text" id="suburb" placeholder="西新宿" disabled>
</form>