Can my common law spouse apply for child benefit from his previous marriage daughter? He had full custody of her started last year. Can we put her on our dependent?

If your spouse's daughter is living with him, he can submit an application for the Child Tax Benefit.  You can also contact TurboTax Live Assist and Review by using the following link, https://turbotax.community.intuit.ca/community/navigating-turbotax/help/i-m-using-turbotax-live-assist-review-how-do-i-talk-to-a-tax-expert/00/873764.