how to make my jhipster entity with one field(singleton style)

I made an entity for my project that holds some system configurations, and want this entity to have only one field i'm thinking that a singleton style would be great but i don't know how i can add it exactly and where since i'm new to jhipster and spring world

i think that if i edit the file 'configuration.model.ts' ( i mean editing the constructor it might work but i'm still not sure if that's it, here is the constructor.

constructor( public id?: number, public letter?: string, )

i'm expecting to have an entity with one field where i can't add more than it and it shows a message in case of someone tried to add a new field to the entity