Hi, try this, first, navigate to your form, and add an HTML embed widget, after the button, but within the form wrapper:
Next paste in the following code to the html embed widget:
<input type="hidden" name="list" value="list-hash-id"/>
<input type="hidden" name="boolean" value="true"/>
Replace list-hash-id with the hashed list ID that you get from Sendy, that points to the email list that user will be subscribing to.
Make sure your page is saved and then republish your site. Test again to see if that corrects the issue. I think you were just missing these extra hidden form fields. To add those hidden fields, we need to use the embed widget at the moment.
I hope that works, let us know. Cheers, Dave