How to define string in quotes in Jetbrains MPS?

Question

I am trying to define JSON language in Jetbrains MPS. I have defined some concepts as

concept ObjectNode extends JsonNode                                                                                                                                                    
                   implements <none>                                                                                                                                                   
                                                                                                                                                                                       
  instance can be root: true 
  alias: <no alias> 
  short description: <no short description> 
 
  children: 
  keyValues : KeyValue[0..n] 

concept KeyValue extends BaseConcept                                                                                                                                             
                 implements <none>                                                                                                                                               
                                                                                                                                                                                 
  instance can be root: false 
  alias: <no alias> 
  short description: <no short description> 
 
 
  children: 
    key : Key[1]        
    value : JsonNode[1]                          

concept Key extends BaseConcept
implements

instance can be root: false alias: short description:

properties: name : string

Now I want to define editor for a key. It should be something double quoted. But I don't understand, how to define it. Whatever I try to enter ny internet searching is rejected by the editor.

What should it be here?

<default> editor for concept Key
  node cell layout:             
     <choose cell model>        
                                
  inspected cell layout:        
    <choose cell model>