// source https://jsbin.com/vopapok

index.html

<style id="jsbin-css">
.page-header {
  position: fixed;
  background: rgba(0, 0, 0, 0.7);
  width: 100%;
  height: 60px;
  z-index: 1000;
}

.page-header select {
  margin: 10px;
  width: 60%;
}

html, body {
  height: 100%;
}
* {padding: 0; margin: 0;}

iframe {
  width: 100%;
  border: none;
  margin-top: 60px;
  height: calc(100% - 60px);
}
</style>
<div class="page-header">
  <select id="links-select"></select>
</div>
<iframe id="offer-target-frame"></iframe>
<script id="jsbin-javascript">
'use strict';

var links = ['https://www.totaljobs.com/job/microwave-engineer/premier-group-uk-job75641731?WT.mc_id=A_RE_JMGPPC_web', 'https://www.cwjobs.co.uk/job/test-engineer/kyna-associates-job75638380?WT.mc_id=A_RE_JMGPPC_web', 'https://www.retailchoice.com/job/junior-quality-engineer/michael-page-job75637336?WT.mc_id=A_RE_JMGPPC_web'];

var select = document.getElementById('links-select');
var iframe = document.getElementById('offer-target-frame');
iframe.src = links[0];

links.forEach(function (link) {
  var option = document.createElement('option');
  option.value = link;
  option.innerHTML = link;
  select.appendChild(option);
});

select.addEventListener('change', function (e) {
  iframe.src = e.target.value;
});
</script>

<script id="jsbin-source-css" type="text/css">.page-header {
  position: fixed;
  background: rgba(0, 0, 0, 0.7);
  width: 100%;
  height: 60px;
  z-index: 1000;
}

.page-header select {
  margin: 10px;
  width: 60%;
}

html, body {
  height: 100%;
}
* {padding: 0; margin: 0;}

iframe {
  width: 100%;
  border: none;
  margin-top: 60px;
  height: calc(100% - 60px);
}</script>

<script id="jsbin-source-javascript" type="text/javascript">const links = [
  'https://www.totaljobs.com/job/microwave-engineer/premier-group-uk-job75641731?WT.mc_id=A_RE_JMGPPC_web',
  'https://www.cwjobs.co.uk/job/test-engineer/kyna-associates-job75638380?WT.mc_id=A_RE_JMGPPC_web',
  'https://www.retailchoice.com/job/junior-quality-engineer/michael-page-job75637336?WT.mc_id=A_RE_JMGPPC_web'
];

const select = document.getElementById('links-select');
const iframe = document.getElementById('offer-target-frame');
iframe.src = links[0];

links.forEach(link => {
  const option = document.createElement('option');
  option.value = link;
  option.innerHTML = link;
  select.appendChild(option);
});

select.addEventListener('change', (e) => {
  iframe.src = e.target.value;
})</script>

jsbin.vopapok.css

.page-header {
  position: fixed;
  background: rgba(0, 0, 0, 0.7);
  width: 100%;
  height: 60px;
  z-index: 1000;
}

.page-header select {
  margin: 10px;
  width: 60%;
}

html, body {
  height: 100%;
}
* {padding: 0; margin: 0;}

iframe {
  width: 100%;
  border: none;
  margin-top: 60px;
  height: calc(100% - 60px);
}

jsbin.vopapok.js

'use strict';

var links = ['https://www.totaljobs.com/job/microwave-engineer/premier-group-uk-job75641731?WT.mc_id=A_RE_JMGPPC_web', 'https://www.cwjobs.co.uk/job/test-engineer/kyna-associates-job75638380?WT.mc_id=A_RE_JMGPPC_web', 'https://www.retailchoice.com/job/junior-quality-engineer/michael-page-job75637336?WT.mc_id=A_RE_JMGPPC_web'];

var select = document.getElementById('links-select');
var iframe = document.getElementById('offer-target-frame');
iframe.src = links[0];

links.forEach(function (link) {
  var option = document.createElement('option');
  option.value = link;
  option.innerHTML = link;
  select.appendChild(option);
});

select.addEventListener('change', function (e) {
  iframe.src = e.target.value;
});