This package contains the types for the Umbraco Backoffice.
npm install -D @umbraco-cms/backoffice
Create an umbraco-package.json file in the root of your package.
{
	"name": "My.Package",
	"version": "0.1.0",
	"extensions": [
		{
			"type": "dashboard",
			"alias": "my.custom.dashboard",
			"name": "My Dashboard",
			"js": "/App_Plugins/MyPackage/dashboard.js",
			"weight": -1,
			"meta": {
				"label": "My Dashboard",
				"pathname": "my-dashboard"
			},
			"conditions": [
				{
					"alias": "Umb.Condition.SectionAlias",
					"match": "Umb.Section.Content"
				}
			]
		}
	]
}
Then create a dashboard.js file the same folder.
import { UmbElementMixin } from '@umbraco-cms/backoffice/element-api';
import { UMB_NOTIFICATION_CONTEXT } from '@umbraco-cms/backoffice/notification';
const template = document.createElement('template');
template.innerHTML = `
  <style>
    :host {
      padding: 20px;
      display: block;
      box-sizing: border-box;
    }
  </style>
  <uui-box>
    <h1>Welcome to my dashboard</h1>
    <p>Example of vanilla JS code</p>
    <uui-button label="Click me" id="clickMe" look="secondary"></uui-button>
  </uui-box>
`;
export default class MyDashboardElement extends UmbElementMixin(HTMLElement) {
	/** @type {import('@umbraco-cms/backoffice/notification').UmbNotificationContext} */
	#notificationContext;
	constructor() {
		super();
		this.attachShadow({ mode: 'open' });
		this.shadowRoot.appendChild(template.content.cloneNode(true));
		this.shadowRoot.getElementById('clickMe').addEventListener('click', this.onClick.bind(this));
		this.consumeContext(UMB_NOTIFICATION_CONTEXT, (_instance) => {
			this.#notificationContext = _instance;
		});
	}
	onClick = () => {
		this.#notificationContext?.peek('positive', { data: { headline: 'Hello' } });
	};
}
customElements.define('my-custom-dashboard', MyDashboardElement);
First install Lit and Vite. This command will create a new folder called my-package which will have the Vite tooling and Lit for WebComponent development setup.
npm create vite@latest -- --template lit-ts my-package
Go to the new folder and install the backoffice package.
cd my-package
npm install -D @umbraco-cms/backoffice
Then go to the element located in src/my-element.ts and replace it with the following code.
// src/my-element.ts
import { LitElement, html, customElement } from '@umbraco-cms/backoffice/external/lit';
import { UmbElementMixin } from '@umbraco-cms/backoffice/element-api';
import { UmbNotificationContext, UMB_NOTIFICATION_CONTEXT } from '@umbraco-cms/backoffice/notification';
@customElement('my-element')
export default class MyElement extends UmbElementMixin(LitElement) {
	private _notificationContext?: UmbNotificationContext;
	constructor() {
		super();
		this.consumeContext(UMB_NOTIFICATION_CONTEXT, (_instance) => {
			this._notificationContext = _instance;
		});
	}
	onClick() {
		this._notificationContext?.peek('positive', { data: { message: '#h5yr' } });
	}
	override render() {
		return html`
			<uui-box headline="Welcome">
				<p>A TypeScript Lit Dashboard</p>
				<uui-button look="primary" label="Click me" @click=${() => this.onClick()}></uui-button>
			</uui-box>
		`;
	}
}
declare global {
	interface HTMLElementTagNameMap {
		'my-element': MyElement;
	}
}
Finally add an umbraco-package.json file in the root of your package folder my-package.
{
	"name": "My.Package",
	"version": "0.1.0",
	"extensions": [
		{
			"type": "dashboard",
			"alias": "my.custom.dashboard",
			"name": "My Dashboard",
			"js": "/App_Plugins/MyPackage/dist/my-package.js",
			"weight": -1,
			"meta": {
				"label": "My Dashboard",
				"pathname": "my-dashboard"
			},
			"conditions": [
				{
					"alias": "Umb.Condition.SectionAlias",
					"match": "Umb.Section.Content"
				}
			]
		}
	]
}