Incorrect (unnecessary use of {{fn}}):

// app/components/search.gjs
import Component from '@glimmer/component';
import { action } from '@ember/object';

class Search extends Component {
  @action
  handleSearch(event) {
    console.log('Searching:', event.target.value);
  }

  <template>
    {{! Wrong - no arguments being bound}}
    <input {{on "input" (fn this.handleSearch)}} />
  </template>
}