There is a new solution that looks much cleaner:
<Form id="myForm">
…
<Modal
...
footer={[
<Button form="myForm" key="submit" htmlType="submit">
Submit
</Button>
]}
>
<CustomForm />
</Modal>
This works because of the Button’s form attribute. Browser support
Original solution’s author: https://github.com/ant-design/ant-design/issues/9380