This example will change the login message.

add_filter( 'frm_global_login_msg', 'change_message' );
function change_message( $message ) {
    $message = 'place new message here';
    return $message;
}

Use this code to customize the Login message in the Formidable > Global Settings → Message Defaults page by adding a link to the login page. To apply the login page link, surround the word or phrase with asterisks. For example: You do not have permission to view this form. *Please sign in to continue*.

add_filter( 'frm_global_login_msg', 'custom_global_login_message' );
function custom_global_login_message( $message ) {
    $frm_settings = FrmAppHelper::get_settings();
    $frm_login_msg = $frm_settings->login_msg;
		
    if ( substr_count( $frm_login_msg, '*' ) == 2 ) {
        $frm_login_msg_array = explode( "*", $frm_login_msg );
			
        $current_url = home_url( add_query_arg( [], $GLOBALS['wp']->request ) ); // get_permalink() doesn't include query vars
			
        $frm_login_msg = $frm_login_msg_array[0];
        $frm_login_msg .= '<a href="' . esc_url( wp_login_url( $current_url ) ) . '" alt="Login">';
        $frm_login_msg .= $frm_login_msg_array[1];
        $frm_login_msg .= '</a>';
        $frm_login_msg .= $frm_login_msg_array[2];
			
	    }
		
    return $frm_login_msg;
}