Hello,

First you need to install Mailchimp plugin. Create a form and then add this form in the sidebar widget. Use the Custom HTML widget and add this code into it:

<div class="boxed-style" style="background:url('https://codeless.co/june/default/wp-content/uploads/2017/11/plants_sidebar.jpg')  no-repeat 100% 100%;">
	<h3 class="widget-title">Join Our Newsletter</h3>
	[mc4wp_form id="403"]
</div>

I already added it for you.
See screenshot: https://postimg.cc/image/r4qmoegxp/
Check it directly in this page: https://dev.painogen.com/careers/

Best regards!