Angular Material: mat-select default value when using reactive forms

You need to define the criteria based on which an object becomes considered as selected using the [compareWith] directive as follows:

<form [formGroup]="parentForm">
      <mat-form-field class="example-full-width">
        <input matInput formControlName="name" placeholder="Name">
      </mat-form-field>

      <mat-form-field class="example-full-width">
        <mat-label>Users</mat-label>
        <mat-select [compareWith]="compareFunction" formControlName="owners" multiple>
          <mat-option *ngFor="let user of users" [value]="user"> {{ user.username }}</mat-option>
        </mat-select>
      </mat-form-field>
</form>

and define your compareFunction:

compareFunction(o1: any, o2: any) {
 return (o1.name == o2.name && o1.id == o2.id);
}