I an Angular 8 project with Angular Material, I have a form and in it I need to add a select box that have this style:

Note the label "Select an option" style how stands in the middle of the line.
The html code:
<mat-form-field>
<mat-label> Select an option</mat-label>
<mat-select>
<mat-option *ngFor="let type of types" [value]="type">{{type}}</mat-option>
</mat-select>
</mat-form-field>
The ts code:
types: string[] = [
'Charmander', 'Bulbasaur', 'Squirtle'
];
I can't find anywhere in the documentation how to make the "Select an option" to stay in the middle of the line. Here is the stackblitz link
Can someone help me please?