Sounds so simple but I've tried quite a few things and none work. 

I'm using Angular 4 and my form is template-driven:

```
<form #form="ngForm" novalidate>
    <label for="insz">{{ 'SEARCH_PAGE.searchInszNumber' | translate }}</label>
    <input type="text" name="insz" [placeholder]="'SEARCH_PAGE.searchInszNumber' | translate" #input required>
    <button (click)="onSearch(input.value)" ><span>{{'SEARCH_PAGE.search' | translate }}</span></button>
</form>

```

I want to disable the button when the (one and only) input field is empty.