W-7 is a form send to IRS for applying ITIN (individual tax identification number) for those individual not qualified for Social security number but still need to file tax. In lieu of SS#, You need this number to file tax or file as dependent. you can download the form from irs.gov.

since you are f2, you likely don't have your own income and the only reason you need to be included in a tax return is that your husband filed you as a dependent.

look carefully at the letter to see what additional documents they request.

Write a letter explaining to them that you don't have SS# but would like to be filed as dependent, and you are currently applying ITIN at IRS.