Typescript - Transform an interface property to Generic

I am using an interface with multiple properties. But in a couple of them, I use any | null, since, the value can be omitted at points.

Example:

export default interface IField {
  fieldName: string;
  fieldType: string;
  fieldLabel: string;
  fieldPlaceholder: string;
  fieldDefaultValue?: any | null; // Transform to Generic

  isHelpAvailable?: string | ReactNode;

  isFieldRequired: boolean;
  isFieldValidated?: Function | boolean;
}

It was proposed to me, to try and use Generics is cases like that. But I haven't used them in the past. Could you please, transform this one for me, as a guide, with a small explanation, on what is going on.

I am using the same pattern in multiple places, but right now, don't know how to tackle them. Thanks!!