How to pre-select an option in mat-autocomplete

Default value?.. well, if you have a default value, the autocomplete will bring you just one option if you set an object to the input value (the default value selected) or a list of possible objects if it is a string. I’m not sure I’m following you… I suppose you have something like this in your code:

<input [formControl]="_inputCtrl" [matAutocomplete]="auto">
<mat-autocomplete #auto>
  <mat-option *ngFor="let option of _options$ | async"
              [value]="option"
              [displayWith]="_displayWithLabel()">
      {{option.label}}
  </mat-option>
</mat-autocomplete>

class MyClass {
  label: string;
  value: any;
}

....

_displayWithLabel = () => (value: MyClass | string): string => {
   return !value 
    ? '' 
    : typeof value === 'string' 
      ? value
      : value.label;
}

....

_options$: Observable<MyClass[]>;
constructor(private _service: MyHttpService) {
  this._options$ = _inputCtrl.valueChanges.pipe(
    startWith(null),
    debounce(300),
    switchMap((value: MyClass | string) => this._getOptions(value))
    takeUntil(this._componentDestroyed$),
  );

  // here you set the default selected value: it should fire the valueChanges event set above and
  // get you a list of options, if there was any based on your default initial value.
  this._inputCtrl.setValue('someDefaultStringToStartWith');
}

private _getOptions(value: MyClass | string): Observable<MyClass[]> {
  if(typeof value === 'string') {
    return this._service.getObjectsByLabel(value);
  } else {
    return of([value])
  }
}

But if you consider the edit page there should be prefilled value. Which the user can remove and then use autocomplete. If the user doesn’t wants to change the value why empty such an input field.

The example above should do it. There’s no need to recreate the input control everytime.

this._inputCtrl.setValue(...) is enough.