I FUCKING DID IT

Added a helper component for managing a list of strings.
This component could, in theory, also be turned into a generic one
for any type of data

frontend-reactive/app/components/auth/registration-form.hbs

@@ -0,0 +1,18 @@
+<div class="registration-form">
+  <p class="registration-form-username">{{this.username}}</p>
+
+  <label>
+    Displayname
+    <Input @type="text" @value={{this.displayname}} placeholder="Displayname" />
+  </label>
+  <label>
+    Description
+    <Input
+      @type="text"
+      @value={{this.description}}
+      placeholder="Account description"
+    />
+  </label>
+  <Util::StringArray @list={{this.gender}} />
+  <p>{{this.extracted}}</p>
+</div>

frontend-reactive/app/components/auth/registration-form.ts

@@ -0,0 +1,21 @@
+import Component from '@glimmer/component'
+import { tracked } from '@glimmer/tracking'
+
+export interface AuthRegistrationFormSignature {
+  // The arguments accepted by the component
+  Args: {
+    username: string
+  }
+  // Any blocks yielded by the component
+  Blocks: {
+    default: []
+  }
+  // The element to which `...attributes` is applied in the component template
+  Element: null
+}
+
+export default class AuthRegistrationForm extends Component<AuthRegistrationFormSignature> {
+  @tracked displayname: string = this.args.username
+  @tracked description: string = ''
+  @tracked gender: Array<{ value: string }> = []
+}