Send Form data to external url

I used to use the following code from within WordPress to let users look up which colleage company they have to contact based on their zipcode:

<form action="https://www.energiewacht.com/action/zipcodes/contactinfo/"
      method="get"
      target="_blank">
Zoek een Energiewacht servicepartner
<input class="search-field" name="zipcode" type="text" value="" placeholder="1234AB" />
<input type="submit" value="Zoek">
</form>

Unfortunately this code no longer works; when I click on “search” I see that the ‘siteurl’, in this case https://geaskb.nl, is placed before the actual link.
Despite searching on w3schools.com, stack exchange etc. I cannot find a suitable solution.

Who can tell me how I can still send the zip code to the external website so that the customer can find the right colleague company there?
The external website uses a zip code module in a different CMS than WordPress.