Put it in Previous surname, with her married surname in Family name. Both are searched, so she can be found either way. Nicknames and other spellings go in Also known as. For example, Mum was born with one surname and took Dad's when they married: put Dad's surname in Family name and hers from birth in Previous surname. If your family prefers, the tree can show birth surnames on the cards too.
Where to find it
Person page → Edit → Essential
Only people with the "Edit" tick.
On a phone: if the card's buttons are tucked away, press ☰ Menu first.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com