How to add new method to existing object?

I tried this but it doesn’t work:

const robot = {
    name: 'Marble',
    model: '5F',
    age: 1,
    
};

robot.prototype.sayHello = function(){return `Hello I am ${this.model}`} 

I get error: Uncaught TypeError: Cannot set properties of undefined (setting ‘sayHello’)