You can add Segment snippet 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 Segment, and copy your segment snippet and paste it in the Segment 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.