I want to put the backup password in a defaults extra file, but I do not know the syntax for doing this. What do I use as the delimiter.

[WHAT IS THE DELIMITER]
password=password

Can someone tell me the syntax?

If you want to put backup user/pass in default extra file you can do like
[client]
user=user
password=password

FYI: For Innobackupex, defaults-file should have client section with above details and mysqld section with datadir option. i.e

[client]
user=user
password=password

[mysqld]
datadir=/var/lib/mysql

niljoshi is correct - but I believe you may have extended or reserved characters in your password. If this is the case, use single quotes to delimit the password.

[client]
password=‘p@ssw0rd!’

I hope this helps . . .

/David C.