Add url param defaultUsername to prefill the login username field

Signed-off-by: David Schilling <dschilling@churchtools.de>

src/components/structures/MatrixChat.tsx

@@ -2090,6 +2090,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
                     onForgotPasswordClick={showPasswordReset ? this.onForgotPasswordClick : undefined}
                     onServerConfigChange={this.onServerConfigChange}
                     fragmentAfterLogin={fragmentAfterLogin}
+                    defaultUsername={this.props.startingFragmentQueryParams.defaultUsername}
                     {...this.getServerProperties()}
                 />
             );