Using triggers, the same as you did in WC3.
There might be some data editor way to get buildings to spawn units for a wars map (like footman frenzy) but for towerdefenses you should still just use triggers.
If you can not make a spawn system by yourself, look at maps like zerg invasion alternative or other defense maps. There are multiple towerdefenses also that have spawn systems you can lear from.
To get them in your editor. Open the editor. Close the default map. Go to File/Open. Choose the battlenet catogry. Log into battlenet using your Battlenet 2.0 account which has your beta key attached to it. Then it should load a huge list of maps to choose from. Choose any defensive map, most maps infact use some kind of spawn system so if you go by name you should easilly find what you are after.