promptComponent.js

import { LightningElement } from 'lwc';
import LightningPrompt from 'lightning/prompt';
export default class PromptComponent extends LightningElement {
    handlePromptClick() {
        LightningPrompt.open({
            message: 'this is the prompt message',
            //theme defaults to "default"
            label: 'Please Respond', // this is the header text
            defaultValue: 'initial input value', //this is optional
        }).then((result) => {
            console.log('Result: '+ result);
            //Prompt has been closed
            //result is input text if OK clicked
            //and null if cancel was clicked
        });
    }
}