First, add a Button Module to the page. 

Next assign a CSS Class in the Advanced settings of the module, for example:

trigger_popup

In case you want to use a simple link somewhere on the page, the CSS Class can be added in text mode of the text editor, like this:

<a href="#" class="trigger_popup">link</a> 

In case if you want to open popup from the menu link you can add Custom Link to the menu and assign CSS class:

You can enable the CSS Class field in the Screen Options.

To create the trigger on click effect, you can use two of the types: the pop up and fly in.

Next, connect your form to an email provider. Without the email provider, account and email list, the form will be inactive. 

This is the tab where we’ll link the opt-in form to one of the buttons. Enable the ‘Trigger on Click’ one. You’ll see that the CSS Selector field will appear immediately, place a custom CSS Class to this field:

.trigger_popup

And enable the opt-in form on everything: 

Note: make sure to add a dot before the class name when you place it to the optin settings.

That's it! Save the settings and test the button that you added to the page.