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 you want to open Bloom popup from the menu link you can add Custom Link to the menu and assign a CSS class.

Go to Screen Options in the right corner, enable CSS Class, and then you can see the "CSS Class" field in each menu item, you can add the class name there for that menu item:

To create the trigger-on-click effect, you can use two of the types: 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. 

You can know more about how to connect Email provider here:

https://www.elegantthemes.com/documentation/bloom/accounts/

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.