You can use the block's own default one by leaving the field blank. This is not without its limitations though .

You really should use your own if at all possible as you will have control of it. If everyone uses the default YouTube key on the block and it exceeds YouTubes daily limit it will max out and the plugin will not display any videos untill the api quota is reset the next day. Note** this is how YouTube works quotas are set and reset daily by them whilst it is unlikely it will max out it is not impossible.

For quickness I recomend you start with the blocks default key then sort your own out later.