title: Custom checkout field enable or disable payment methods in Woocommerce 3
body: <p>So i'm working on <a href="http://www.lichtunie.nl" rel="nofollow noreferrer">http://www.lichtunie.nl</a>
  We have a functioning checkout page with the needed fields. 
  The problem is this:
  In the Netherlands (where we're based) We have something called KvK, if you start a company you need to register it there and you get a KvK number. 
  We can check those numbers through a website to see if they're legit and how their payment history is. </p>
  
  <p>Now we have the option of "paying with cheque" Which let's you order and pay within a 30 day time period after receiving the invoice. 
  What we want now is that when someone doesn't fill in their KvK number field on checkout they can't use this method of payment. </p>
  
  <p>As soon as they've filled in the "KvK number" field they should be able to though.</p>
  
  <p>I've been looking for a while and just can't figure out how to do this. Anyone got any tips?</p>
  
  <p>Thanks in advance,</p>
  
  <p>Lex</p>