Fill form into Iframe
Node.js + Puppeteer
(async () => {
    const websiteUrl = 'https://example.com'; // replace with your website url
    const browser = await puppeteer.launch();
    const page = await browser.newPage();
    await page.goto(websiteUrl); // 

    const frameHandle = await page.waitForSelector('#iframe');
    const frame = await frameHandle.contentFrame();
    const fieldSelector = "input[name='username']"; // replace with your field selector into the iframe
    await frame.waitForSelector(fieldSelector);
    const userNameValue = 'username'; // replace with your username value
    await frame.type(fieldSelector, userNameValue);

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

PHP + PuPHPeteer
<?php
use Nesk\Puphpeteer\Puppeteer;

$puppeteer = new Puppeteer;
const WEBSITE_URL = 'https://example.com';
$browser = $puppeteer->launch();
$page = $browser->newPage();
$page->goto(WEBSITE_URL); // replace with your url

$page->waitForSelector('#iframe');
$frame = $page->frames()[1]; // Suppose  that the iframe is the second frame
$fieldSelector = "input[name='username']"; // replace with your field selector into the iframe
$userNameValue = "username"; // replace with your username value
$frame->waitForSelector($fieldSelector);
$frame->type($fieldSelector, $userNameValue);

$browser->close();