If another antivirus product is installed and working correctly, Microsoft Defender Antivirus will disable itself. The Windows Security app will change the Virus & threat protection section to show status about the AV product, and provide a link to the product's configuration options.
Underneath any third party AV products, a new link will appear as Microsoft Defender Antivirus options. Clicking this link will expand to show the toggle that enables limited periodic scanning. Note that the limited periodic option is a toggle to enable or disable periodic scanning.
Sliding the switch to On will show the standard Microsoft Defender Antivirus options underneath the third party AV product. The limited periodic scanning option will appear at the bottom of the page.