Angular 6 - Unable to run external functions inside a particular function

You are passing a reference to an object method, but the value of this is not being set. So you need to bind(this) to the function reference.

public addTagNowRef: (name)=>void;

constructor(public afs: AngularFirestore) {
  this.categoriesCollection = this.afs.collection('categories', ref => ref.orderBy('title', 'asc'));
  this.addTagNowRef = this.addTagNow.bind(this);
}

Then use that property in the template.

<ng-select [(ngModel)]="client.categoryId"
           class="form-control"
           [ngClass]="{'is-invalid':clientCategoryId.errors && clientCategoryId.touched}"
           #clientCategoryId="ngModel"
           name="categoryId"
           [addTag]="addTagNowRef"
           required>
  <ng-option *ngFor="let cat of cats" [value]="cat.id">{{cat.title}}</ng-option>
</ng-select>

Alternatively, you can use an arrow function to forward the call to the method.

public addTagNowRef: (name)=>void;

constructor(public afs: AngularFirestore) {
  this.categoriesCollection = this.afs.collection('categories', ref => ref.orderBy('title', 'asc'));
  this.addTagNowRef = (name) => this.addTagNow(name);
}

The point here is that this must reference the component.