How can I add a custom field in registration form with image

I wanted to add a custom field in my registration form.

So it works, it looks as follows. The passwortfield has the little image, my new field location doesn’t.

I added the field via Register/Modal/Expressions>Pro

<div class="fieldset fieldset--city">
      <label class="image-replace lrm-email lrm-ficon-location" title=""></label>
      <input name="city" class="full-width has-padding has-border" type="text" placeholder="City" required="">
      <span class="lrm-error-message"></span>
</div>

This was a snippet from the Pluginsite, so how can I get the lrm-ficon-location?