typed-form-submission.tsx

// Define which elements this form is going to have
interface FormElements extends HTMLFormControlsCollection {
  fieldName: HTMLInputElement;
}

// Set a "HTMLFormElement" interface with those elements
interface MyForm extends HTMLFormElement {
  readonly elements: FormElements;
}

function Foo() {
  return (
    <form onSubmit={(event: React.SyntheticEvent<MyForm>) => {
      // form elements are going to be defined under:
      event.currentTarget.elements;

      // get value of a field
      event.currentTarget.elements.fieldName.value;
    }}>
      {/* ... */}
    </form>
  );
}