For some input fields (property) we use an input placeholder which suggests the kind of input needed,

but for add value statement, we don't.

Suggestion: Use input placeholder for value input, too. (or, more general: Use input placeholder for all input fields)

Simple solution: Use the name of data in the datastructure, like "property", "value" etc.
Advanced: Use the data structure to give further help; e.g. if we edit "Harry Potter", and add the property "from fictional universe", the input field could state "Fictional universe Harry Potter is from"

Applies to:

• Add value statement to existing property

General Principles: »I want to know what an interaction causes« (principle of least surprise) and »stick to standards«