You must enter your social security number in the form ###-##-####.
Just a note about this, social security number equivalents in other countries
are not in the same form as is the one used in United States.
For example, in Finland the "personal identification number" is in DDMMYYCZZZQ form, where DDMMYY is birthdate, C is the century identification sign (+ for the 19th century, - for the 20th and A for the 21st), ZZZ is the personal identification number and Q is a checksum character. For example, a valid number would be 311290-999K.
Just thought I'd mention this.