title: Hide submit button for PayPal payment form if input field is not 40 characters
body: <p>I have a PayPal buy now button form on my website that users fill out to purchase a product.   </p>
  
  <pre><code>&lt;form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top"&gt;
  &lt;input type="hidden" name="cmd" value="_s-xclick"&gt;
  &lt;input type="hidden" name="hosted_button_id" value="BHQLX7V2W2GJQ"&gt;
  &lt;table&gt;
  &lt;tr&gt;&lt;td&gt;&lt;input type="hidden" name="on0" value="Select package"&gt;Select package&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td&gt;&lt;select name="os0"&gt;
      &lt;option value="Premium"&gt;Premium $5.49 USD&lt;/option&gt;
      &lt;option value="Platinum"&gt;Platinum $14.49 USD&lt;/option&gt;
      &lt;option value="Diamond"&gt;Diamond $9.49 USD&lt;/option&gt;
  &lt;/select&gt; &lt;/td&gt;&lt;/tr&gt;
  &lt;tr&gt;&lt;td&gt;&lt;input type="hidden" name="on1" value="Enter your 40 character UDiD"&gt;Enter your 40 character UDiD&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td&gt;&lt;input type="text" name="os1" id="field" maxlength="40" placeholder="Paste UDiD here"&gt;&lt;/td&gt;&lt;/tr&gt;
  &lt;/table&gt;
  &lt;input type="hidden" name="currency_code" value="USD"&gt;
  &lt;input type="image" src="https://www.paypalobjects.com/en_AU/i/btn/btn_buynow_LG.gif" border="0" name="submit" id="paypalButton" alt="PayPal — The safer, easier way to pay online."&gt;
  &lt;img alt="" border="0" src="https://www.paypalobjects.com/en_AU/i/scr/pixel.gif" width="1" height="1"&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>In this form, there is a text input field:</p>
  
  <pre><code>&lt;input type="text" name="os1" id="field" maxlength="40"&gt;
  </code></pre>
  
  <p>I want to force the user to enter their exactly 40 character UDiD (for their iOS device) before they can purchase the product by clicking the 
  'Buy now' button (submit button for the form). How can I do this by disabling/hiding the button if the input field does not have a 40 character string entered?</p>
  
  <p><strong>Edit: new form code</strong></p>