To create VAST tag please go to Ads page
On the Ads page, you will have a list of players in your account.
To create a new Ad tag click Create Ad Tag button.
For ID enter a name and for Tag URL enter link to your ad tag.
You can find sample Vast tags on google page for vast tags
If you are doing it via SDK please visit https://publit.io/docs/#create-adtag for full information.