Give placeholder to select in Angular 6

simply use [value]="undefined"

<select class="form-control" [(ngModel)]="selectedCategory">
    <option [value]="undefined" disabled hidden>Select...</option>
    <option *ngFor="let c of categories">{{c}}</option>
</select>

for a template driven angular form you may want to consider this demo. you can find the code here

for a simple html form here is a snippet.

<form>
    <select required>
        <option value="" disabled selected hidden>Select a value</option>
        <option value="0">option 1</option>
        <option value="1">option 2</option>
    </select>
</form>

<option value="" disabled selected hidden>Select your option</option>

You can use [value]="" selected hidden

I have create a demo on Stackblitz

<form role="form" class="form form-horizontal" (ngSubmit)="onSubmit()" #form="ngForm" ngNativeValidate>
    <div class="form-group row">
        <div class="col-xl-4 col-lg-6 col-md-12">
            <fieldset class="form-group">
                <label for="customSelect">Categories:</label>
                <select class="custom-select d-block w-100" id="Category" [(ngModel)]="Category" name="Category" required placeholder="d.ff">
                    <option hidden [value]=""  selected>Select one category </option>
                    <option *ngFor="let item of myBusinessList" [value]="item.id">{{item.name}}</option>
                </select>
            </fieldset>
        </div>
    </div>
    <button type="submit" class="btn btn-raised btn-danger">Save</button>
</form>

If you want the first value to be selected when Category is still undefined, you can assign the value undefined to the first option with ngValue:

<option [ngValue]="undefined" hidden>Select one category</option>