Place a misc_mission entity at the bomb spot and set the item property to the item that must be placed there (this can also be used to deliver something). This item id comes from the ufo script files and must be a valid item-definition id. (use e.g. the existing bomb types, bomb1 or bomb2.
Setting the time of the misc_mission entity defines the amount of rounds the item will wait for the explosion. Once this time is due the mission target is won, if no other mission is in the map, the map is won for the team that had to place the item.
If you need an item, you should be aware that it's not enough for skirmish or multiplayer to have this item in a campaign-only game. You should either be aware that you are building a campaign-only-map or place the item somewhere into the map by placing a misc_item entity.