How to submit form from a button outside that component in React?

You can achieve this by using regular HTML capabilities (HTML form Attribute), no need to use the React hacks:

Add “id” attribute to your form: id=’my-form’

class CustomForm extends Component {
    render() {
        return (
             <form id='my-form' onSubmit={alert('Form submitted!')}>
                // Form Inputs go here    
             </form>
        );
    }
}

Then add the same Id to the “form” attribute of the target button outside of the form:

<button form='my-form' type="submit">Outside Button</button>

Now, the ‘Outside Button’ button will be absolutely equivalent as if it is inside the form.

Note: This is not supported by IE11.