Fill out your timeline. Did she arrive on CR-1 or adjust status? Either way, she should already have a myUSCIS account. If arrived on CR-1, she would've made it after her consulate interview when paying the ELIS fee. Also, for my wife and my bro's wife who both came on CR-1, the USCIS Online Account number was visible under the "Profile" section but then that entire section vanished. So unless you wrote it down, it's gone until you file another case. I wrote my wife's down. My brother didn't. So they had to leave it blank on the I-751. Either way, it's not a big deal. They will give you a number and instructions if you leave it blank.