In order to use one-time notifications, you'll have to request permission to use the feature with your Facebook page first.
Here's where you can find that setting for your Facebook page:
Navigate to your Facebook page.
Settings > Advanced Messaging.
Scroll down until you find Requested Features.
Here, you'll be able to request permission to use one-time notifications for your Facebook page. Permission is granted per page, so this does have to be done for each page you'd like to use one-time notifications with.
One your page is granted permission, the one-time notification option should match the approved state below:
Important note: pages that receive high volumes of negative user feedback, or that confuse, deceive, defraud, mislead, or negatively surprise users.
Your one-time notifications can include promotional content. However, they should avoid violating these Facebook policies:
Don’t confuse, deceive, defraud, mislead, spam or surprise anyone. Make sure that there is a clear and obvious relationship between the prompt, and the content of the message. The content of the one-time notification request and the message must comply with community standards.
Don’t incentivize this action. Don’t promise users money, or equivalent to sign up for messages.