src/vs/workbench/contrib/webview/browser/webviewElement.ts/IFrameWebview/initElement
class IFrameWebview:	protected initElement(extension: WebviewExtensionDescription | undefined, options: WebviewOptions, extraParams?: { [key: string]: string }) {
		const params: { [key: string]: string } = {
			id: this.id,
			swVersion: String(this._expectedServiceWorkerVersion),
			extensionId: extension?.id.value ?? '', // The extensionId and purpose in the URL are used for filtering in js-debug:
			...extraParams,
			'vscode-resource-base-authority': this.webviewRootResourceAuthority,
		};

		if (options.purpose) {
			params.purpose = options.purpose;
		}

		const queryString = (Object.keys(params) as Array<keyof typeof params>)
			.map((key) => `${key}=${encodeURIComponent(params[key]!)}`)
			.join('&');

		this.element!.setAttribute('src', `${this.webviewContentEndpoint}/index.html?${queryString}`);
	}