JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="input1" data-site="site.ru" data-rubric="catalog" data-pages="1-15">
<input type="text" id="input2">

JavaScript

input1.addEventListener('input', function(e) {
	var site = e.target.dataset.site;
  var rubric = e.target.dataset.rubric;
  var pages =  e.target.dataset.pages;
  
	var str = site + '/' + rubric + '/' + e.target.value + '/' + pages;
  input2.value = str;
  if(e.target.value) {
  	input2.value = str;
  }else {
  	input2.value = '';
  }
})