Whether or not you can utilize the Pomodoro timer depends on the Toggl Track app you are using. The feature is currently available in these apps:
Desktop app (both Windows and macOS)
Browser extension (both Firefox and Chrome)
Enabling Pomodoro in the Windows desktop app
Click on the cogwheel icon > Preferences (or hit CTRL + ,).
Enable "Pomodoro timer" from the "General" tab.
Enabling Pomodoro in the macOS desktop app
Click on Toggl Track (in the main menu) > Preferences (or hit ⌘ ,).
Enable the "Pomodoro timer" option from the "General" tab.
Enabling Pomodoro in the browser extensions
Click on the Toggl Track extension icon and then on the cogwheel icon.
Navigate to "Pomodoro" in the sidebar on the left.
Under "Pomodoro settings" tick the "Enable Pomodoro" option.
As of this moment, the Pomodoro timer is not available in these apps:
Toggl Track web app
If you'd like to submit an official feature request for including Pomodoro into the web app and/or mobile apps, send an email to Toggl Track Support or start a conversation by clicking on the chat icon in the bottom right corner.