Hi !
It should be #share2steem in the description. share2steem in the Tags is optional.
But know that the tags that will be converted to Steem tags are the real "Tags" from your video (not the hashtags that will be in your description).
We need to proceed as such because YouTube indexing can be slow when you use only tags, whereas with a hashtag in the description, ths indexing is almost instant. With only the tag, there's a high risk your video will be cross-posted 24 to 48 hours later (but it will work but with a really big delay)