Allow app to send form data

Hello. I’m making an app which uses a third party service through iframe and API.

For this purpose I’ve had problems with content-security-policy. Previously I’ve been able to get through all this by adding this to my app:

$policy = new ContentSecurityPolicy();

However I still have one error in my way:

Refused to send form data to '' because it violates the following Content Security Policy directive: "form-action 'self'".

What is the correct to let an nextcloud app send form data to another domain?