select the first item in dropdown when there is only one item, and disable the dropdown primeng

If I have the following dropdown, how can I select the first item if there is only 1 element in the filteredGroupNames then disable the dropdown and don't display the placeholder

<ng-template pTemplate="content">
<div class="field">
<label>Name</label>
<p-dropdown [(ngModel)]="newOrSelectedGroupName" [options]="filteredGroupNames" [showClear]="true"
placeholder="Select a group"

></p-dropdown>
</div>
</ng-template>

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