room-join.component.html

<form (ngSubmit)="joinRoom(roomId.value)">
    <div fxLayout="row" fxLayoutAlign="center" fxLayoutGap="10px">
      <mat-form-field class="input-block">
        <input matInput #roomId placeholder="Room-Id" [formControl]="roomFormControl"
               [errorStateMatcher]="matcher" maxlength="8"/>
        <mat-hint align="end">{{roomId.value.length}} / 8</mat-hint>
        <mat-error *ngIf="roomFormControl.hasError('required')">Please enter a room-id.</mat-error>
        <mat-error *ngIf="roomFormControl.hasError('minlength')">A room-id has exactly 8 digits.</mat-error>
      </mat-form-field>
      <button mat-fab color="primary" type="submit">
        <mat-icon>send</mat-icon>
      </button>
    </div>
</form>
<button mat-button color="primary" (click)="joinDemo()">Join demo room <mat-icon>lightbulb_outline</mat-icon></button>