How to fix \useshorthands \defineshorthand usage on bookmark entries?

\documentclass[10pt,a5paper]{abntex2}
\usepackage[brazil]{babel}
\usepackage{hyphenat}

\useshorthands{~}\defineshorthand{~=}{\hyp{}}

\begin{document}

\chapter{First~=Chapter}

\end{document}

Instead of ~= to become - (hyphen) on my bookmarks entries, it become = (a space and an equal sign):

hyperref doesn't let you do that, apparently. A bit hackish, but try this (this assumes that whatever ~ in a section title will become a valid pdf string):

\documentclass[10pt,a5paper]{abntex2}
\usepackage[brazil]{babel}
\usepackage{hyphenat}

\makeatletter
\def\HyPsd@LetUnexpandableSpace#1{%
\if\noexpand~\noexpand#1%
\else
\expandafter\@firstofone
\fi{\expandafter\futurelet\expandafter#1\expandafter\@gobble\space\relax}}
\makeatother

\useshorthands{~}
\defineshorthand{~=}{\hyp{}}

\begin{document}

\chapter{First~=Chapter}

\end{document}