To control whether or not the space is added with fast tokenizers, you need to wrap it in an AddedToken:
from transformers import AddedToken

tokenizer_fast.add_tokens(AddedToken("<NEW_TOKEN>", lstrip=True))

You can also choose if you want to remove or not the space after with the rstrip argument.
@s4sarath Your remark is completely out of order, especially since BART uses a byte-level BPE tokenizer.