
Add new entry is only in the menu when the card has at least one property with Allow enrichment turned on. On a read-only card there is nothing a hand-made entry could hold, so the action is absent.
1
Choose Add new entry
Studio asks for the identifier of the new entry first.

2
Give the entry an id
The id must be unique on this card and may hold only letters, numbers,
., _ and -, up to 250 characters. id itself is reserved. Studio checks the id against the existing entries and refuses one that is already taken.This id is what relations, segments and destinations refer to, and it cannot be changed afterwards.3
Fill in the values
Add opens the entry full screen with every value empty.
The property list starts filtered to Property status: Editable, since those are the only properties you can fill. Switch it to All to see the sourced properties too — they stay read-only and empty.

4
Save
Studio closes the entry, and the new entry is in the list.