Video Advertisement can be enabled in two steps: configuring the video at Control Center and whitelisting the necessary domains on the router.
Configure the video at Control Center
Under Manage / Location / Location Name / Modify HotSpot Data and Settings select Video Advertisment & Splash Page Lock
Tick "video advertisement enabled". Make sure that you are using Responsive Skin. This can be checked at at "Splash Page Settings" in this same "Modify Hotspot Data and Settings" menu.
Select your video and copy the video code.
This means that if your favourite video's URL is this https://www.youtube.com/watch?v=lNS32DRc01c, paste the last segment of it there. This is the one after "watch?v=" so in our case: "INS32DRc01c".
Finally, set the number of seconds after which video can be skipped.
2. Whitelist the necessary domains on the router.
As all the access to the internet is blocked before authentication, certain domains need to be whitelisted for additional functionalities. If help is needed, use this article about how to whitelist domains.
The domains to be whitelisted:
Mikrotik routers need white listing a little special way. You can read details in this article.
Once done, end result on the splash page should look like this: