Creating a Zone
First, create a zone from the Tapsell panel of type VAST (preroll) and use the zoneId when requesting or showing an ad.
Requesting and Showing Ads
To show VAST ads you need to use the Google IMA library. You can read the IMA documentations at this link.
To show ads in IMA you need an ad tag url. You can use the following lines to get your ad tag url:
Tapsell.getVastTag(ZONE_ID)
You can do the rest of the implementation following the IMA documentations.