Apparently it is not possible to do with the tb-entity-subtype-select entity.
However, I was able to do it by just using mat-form-field. And it works just fine
<mat-form-field fxFlex class="mat-block">
<mat-label>Title</mat-label>
<mat-select
fxFlex
class="mat-block"
formControlName="formName"
[required]="true"
[showLabel]="true"
>
<input matInput formControlName="formName" required>
<mat-option value="Option 1">Option 1</mat-option>
<mat-option value="Option 2">Option 2</mat-option>
</mat-select>
</mat-form-field>