proper event annotation

Author: yaya-usman

src/components/structures/auth/Registration.tsx

@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import { createClient } from 'matrix-js-sdk/src/matrix';
-import React, { ReactNode } from 'react';
+import React, { ChangeEvent, FormEvent, ReactNode } from 'react';
 import { MatrixClient } from "matrix-js-sdk/src/client";
 import classNames from "classnames";
 import { logger } from "matrix-js-sdk/src/logger";
@@ -147,7 +147,7 @@ export default class Registration extends React.Component<IProps, IState> {
         window.removeEventListener("beforeunload", this.unloadCallback);
     }
 
-    private unloadCallback = (event) => {
+    private unloadCallback = (event: BeforeUnloadEvent) => {
         event.preventDefault();
         event.returnValue = "";
         return "";