Formly - How can I link the field to the model of a custom template - angular8

Question

I am using Angular 8 with Formly for creating a select input type with search. As a solution, I found that ng-select will suit me well.
The question is how can I link the [model] directive to the Formly field which uses my custom template ?
Note: I do not want to use a plain text, like model.my.special.key - I need to give it something general. This way I can use it with other fields that I want to make use of this selector with search.
In more details what I did so far:
I have created a new #Component that extends FieldType and has a template that uses the selector for displaying the options it receives as input
#Component({
selector: "SelectorWithSearchModule",
template: `
<ng-select
[items]="to.options"
bindLabel="label"
bindValue="code"
labelForId="code"
placeholder="{{ to.label }}"
[multiple]="to.multiple"
clearAllText="Clear"
(change)="onChange($event)"
[(ngModel)]="what-to-put-in-here?"
>
</ng-select>
`
})
export class SirutaModule extends FieldType implements OnInit {}
I have declared this new Component in app.module.ts and extented the Formly types with it:
eg:
import { SelectorWithSearchModule } from "./types/selector/special-selector.component"; // declaration
...
FormlyModule.forRoot({ // from here begins the Formly definition
types: [
{
name: "special-selector",
component: SelectorWithSearchModule
}
]
I am using the special-selector name when defining new Formly fields
{
className: "col-8",
type: "special-selector",
key: "my.hierarchy.key",
templateOptions: {
label: "some label here",
options: ["option_one", "option_two"]
}
}
I want that in the end, the key my.hierarchy.key to contain the value user selected in my custom template, but it is not very clear how.