In the first instance, you’ll need to arrange for your marketing sign-up option to be shown at checkout.
To do this, visit:
Settings > Checkout and scroll down to find Form options
Change the Shipping address phone number to Optional.
Then, save the changes by clicking the Save button at the top of that page.
Scroll down to find Email Marketing.
Ensure that Show a sign-up option at checkout is checked and that the Preselect the sign-up option isn’t checked. See the screenshot below.
Save your changes by clicking the Save button located at the top of this page.