You can add Google Tag Manager (GTM) codes to your campaign by clicking the 'Settings' on the top right corner of your dashboard.
(Note: this article is only applicable for landing pages, forms, bonus entry sweepstakes. It doesn't apply to other social media contests like photo Contest, video contest, etc)
Select Google Tag manager
Copy your Google Tag Manager ID and paste it in the text box
If you want it added to both the Main Page and Thank You page be sure to check this option here.
Alternatively you can add a different code to to the Thank You page by clicking the tab here
Then just click Save and your code will be added to your page.