To add a MailChimp subscription in sidebar you must follow the next steps:

1. Install and activate MailChimp For WordPress plugin.

2. Now it will appear in wordpress menu section like in below screenshot.

3. To activate MailChip, you need to connect it from your MailChip account with API Key. Just follow steps in MailChip area to get your API Key.

4. Go to MailChimp For Wp -> Form and add the Newslatter Form Code.

<h6 class="c-mailchimp__desc">Get Interesting News</h6>
<p class="c-mailchimp_short">
Receive the latest news via email.
<p class="c-mailchimp__email">
<i class="far fa-envelope"></i>
<input type="email" name="EMAIL" placeholder="E-mail" required />
<p class="c-mailchimp__submit">
<input type="submit" class="c-button" value="Subscribe" />

5. The last step is to add the widget into sidebar. In Appearance -> Widgets drag and drop the MailChimp Sign-Up like in the following screenshot.