Puppeteer - How to fill form that is inside an iframe?

I figured it out myself. Here's the code.

console.log('waiting for iframe with form to be ready.');
await page.waitForSelector('iframe');
console.log('iframe is ready. Loading iframe content');

const elementHandle = await page.$(
    'iframe[src="https://example.com"]',
);
const frame = await elementHandle.contentFrame();

console.log('filling form in iframe');
await frame.type('#Name', 'Bob', { delay: 100 });

Instead of figuring out how to get inside the iFrame and type, I would simplify the problem by navigating to the IFrame URL directly

https://warranty.goodmanmfg.com/registration/NewRegistration/NewRegistration.aspx?Sender=Goodman

Make your script directly go to the above URL and try automating, it should work

Edit-1: Using frames

Since the simple approach didn't work for you, we do it with the frames itself

Below is a simple script which should help you get started

const puppeteer = require('puppeteer');
(async () => {
    const browser = await puppeteer.launch({ headless: false });
    const page = await browser.newPage();
    await page.goto('http://www.goodmanmfg.com/product-registration', { timeout: 80000 });
    var frames = await page.frames();
    var myframe = frames.find(
        f =>
            f.url().indexOf("NewRegistration") > -1);
    const serialNumber = await myframe.$("#MainContent_SerNumText");
    await serialNumber.type("12345");

    await page.screenshot({ path: 'example.png' });

    await browser.close();
})();

The output is