You enter playtest mode by pressing F10 on your keyboard. The loadout screen opens and elements set to Execute on Startup will be executed. You can buy assets, enter PrePlanning if set up, change weapons and music just as if you opened the level through Ready up to get spawned in the level.
You can run around, shoot and interact as usual and return to editor mode at any time by pressing F10 again, while the game continues to run in the background.
While building your level layout, it’s fine to save while a playtest is running. However it’s highly advised not to do so in later stages with navigation and logic set up, as it has a high chance of breaking.

Sometimes elements don't properly link to each other unless you reload the level. If you want to test a script you should save and reload the level before testing to make sure the elements are actually linked.