How to show MailChimp form in a Lightbox. https://sridharkatakam.com/how-to-show-mailchimp-form-in-a-lightbox/

gistfile1.html

<a href="#mc_form_pop" class="fancybox-inline button">Your link/button text</a>

<div style="display:none" class="fancybox-hidden">
	<div id="mc_form_pop">

		<!-- Begin MailChimp Signup Form -->
		
		<!--End mc_embed_signup-->

	</div>
</div>

gistfile2.html

<a href="#mc_form_pop" class="fancybox-inline button">Your link/button text</a>

<div style="display:none" class="fancybox-hidden">
	<div id="mc_form_pop">

		<!-- Begin MailChimp Signup Form -->
		<link href="//cdn-images.mailchimp.com/embedcode/classic-081711.css" rel="stylesheet" type="text/css">
		<style type="text/css">
			#mc_embed_signup{background:#fff; clear:left; font:14px Helvetica,Arial,sans-serif; }
			/* Add your own MailChimp form style overrides in your site stylesheet or in this style block.
			   We recommend moving this block and the preceding CSS link to the HEAD of your HTML file. */
		</style>
		<div id="mc_embed_signup">
		<form action="//SridharKatakam.us5.list-manage.com/subscribe/post?u=8cf0a9ef20395132aa021c615&amp;id=2c3c7ab970" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" class="validate" target="_blank" novalidate>
			<div id="mc_embed_signup_scroll">
			<h2>Subscribe to our mailing list</h2>
		<div class="indicates-required"><span class="asterisk">*</span> indicates required</div>
		<div class="mc-field-group">
			<label for="mce-FNAME">First Name </label>
			<input type="text" value="" name="FNAME" class="" id="mce-FNAME">
		</div>
		<div class="mc-field-group">
			<label for="mce-LNAME">Last Name </label>
			<input type="text" value="" name="LNAME" class="" id="mce-LNAME">
		</div>
		<div class="mc-field-group">
			<label for="mce-EMAIL">Email Address  <span class="asterisk">*</span>
		</label>
			<input type="email" value="" name="EMAIL" class="required email" id="mce-EMAIL">
		</div>
			<div id="mce-responses" class="clear">
				<div class="response" id="mce-error-response" style="display:none"></div>
				<div class="response" id="mce-success-response" style="display:none"></div>
			</div>    <!-- real people should not fill this in and expect good things - do not remove this or risk form bot signups-->
			<div style="position: absolute; left: -5000px;"><input type="text" name="b_8cf0a9ef20395132aa021c615_2c3c7ab970" tabindex="-1" value=""></div>
			<div class="clear"><input type="submit" value="Subscribe" name="subscribe" id="mc-embedded-subscribe" class="button"></div>
			</div>
		</form>
		</div>
		<script type='text/javascript' src='//s3.amazonaws.com/downloads.mailchimp.com/js/mc-validate.js'></script><script type='text/javascript'>(function($) {window.fnames = new Array(); window.ftypes = new Array();fnames[1]='FNAME';ftypes[1]='text';fnames[2]='LNAME';ftypes[2]='text';fnames[0]='EMAIL';ftypes[0]='email';}(jQuery));var $mcj = jQuery.noConflict(true);</script>
		<!--End mc_embed_signup-->

	</div>
</div>

style.css

@media only screen and (min-width: 481px) {
	#mc_form_pop {
		width: 460px;
	}
}