Preview: Add allow-forms to iframe sandbox attributes (#20701)

Add 'allow-forms' to iframe sandbox attributes

Author: rickbutterfield

src/Umbraco.Web.UI.Client/src/apps/preview/preview.element.ts

@@ -65,7 +65,7 @@ export class UmbPreviewElement extends UmbLitElement {
 						src=${this._previewUrl}
 						title="Page preview"
 						@load=${this.#onIFrameLoad}
-						sandbox="allow-scripts allow-same-origin"></iframe>
+						sandbox="allow-scripts allow-same-origin allow-forms"></iframe>
 				</div>
 			</div>
 			<div id="menu">