Tip: In the block category "Form" there is a CMS element "Form" which can be used. Select the "Newsletter" form type in the element settings.
Assigning the page
Switch to the "Basic information" settings of your store via the main navigation ("Settings" => "Shop" => "Basic information")
Scroll down a bit to the section "Shop pages".
Select the layout created above at "Shop page layout for newsletter pages".
Confirm this change by clicking "Save".
Screenshots
Settings > Basic information
Further information
Note: How to make a layout available in the storefront by using a navigation is described in the following tutorial: Linking shop pages in the storefront