How do I access string keys when using Typescript? [closed]

Let’s say I have the following model definition:

export interface formData {
    formSection: {},
    errors: formError[]
}

export interface formError {
    [key: string]: {
        'default'?: string
        '201'?: string
    }
}

and I receive a JSON like this:

{
  "formSection": {},
  "errors": {
    "firstName": {
      "default": "invalid first name",
      "201": "first name has invalid characters"
    }
  }
}

how can I access errors.firstName.default when TS does not know about the firstName key existence? If I do something like:

const myFunction = (param: formData) => {
    return (
        <>
            {param.errors.firstName.value}
        </>
    )
}

TS will complain saying firstName does not exist in the model, which is correct, but then how can I access that “dynamic” key?