Once you've made the purchase you will receive an email with the key to activate your product.
Activating Office 2019 by Phone
Here's a step-by-step guide on how to activate Office 2019 using the phone:
Open an Office Application: Launch any Office 2019 application (e.g., Word, Excel, PowerPoint).
Access Account Settings:
Word, Excel, PowerPoint, Outlook, Publisher, and Access: Go to File > Account.  
1. Access your Account Privacy Settings - Microsoft Support
support.microsoft.com
Visio and Project: Go to Help > Activate Product.
Choose Phone Activation: Select the option labeled Activate Product or Activate by Phone.
Note the Installation ID: You'll be provided with a unique installation ID. Make sure to write it down or have it ready.
Call the Activation Center: Dial the phone number provided for your region. You can find this number on the Microsoft website or in your Office product documentation.
Follow the Automated Instructions: The automated system will guide you through the activation process. You'll need to enter your installation ID when prompted.
Provide Confirmation Code: Once the activation is complete, you'll receive a confirmation code. Enter this code into the designated field in your Office application.
Verify Activation: Your Office 2019 should now be activated. You can check the status by going back to the Account settings.