'handleSubmit' is not defined no-undef

Here I am getting an error of handleSubmit function because ,here I am using this syntax 'function PostForm () ' instead of 'class PostForm extends Component {}',so what needs to be changed in order to work with handlesubmit function.

function PostForm () {
       handleSubmit = (e) => {
    e.preventDefault();
    const title = this.getTitle.value;
    const message =  this.getMessage.value;
    const data = {
      id: new Date(),
      title,
      message
    }
    console.log(data)
  }

return (
<div>
  <h1>Create Post</h1>
  <form onSubmit={this.handleSubmit}>
   <input required type="text" ref={(input)=>this.getTitle = input} 
    placeholder="Enter Post Title"/>
   <br /><br />
   <textarea required rows="5" ref={(input)=>this.getMessage = input} cols="28" 
    placeholder="Enter Post" />
   <br /><br />
   <button>Post</button>
  </form>
</div>
);

}
export default PostForm;