documentclass{book}
usepackage{amsthm}
usepackage{enumitem}
usepackage{xparse}
usepackage{hyperref}

usepackage{lipsum}

newtheoremstyle{fctaylor}% name
  {topsep}%      Space above
  {topsep}%      Space below
  {normalfont}%         Body font
  {}%         Indent amount (empty = no indent, parindent = para indent)
  {bfseries}% Thm head font
  {}%        Punctuation after thm head
  {0pt}%     Space after thm head: " " = normal interword space;
  {makethmhead{#1}{#2}{#3}}

newlengthfctaylortheoremindent
AtBeginDocument{setlengthfctaylortheoremindent{3em}} % <- customize here
newlengthfctaylorlabelsep
AtBeginDocument{setlengthfctaylorlabelsep{1em}} % <- customize here

makeatletter
newcommand{makethmhead}[3]{%
  gdefthisthmhead{%
    makebox[fctaylortheoremindent][l]{bfseries#2}%
    {bfseries#1}%
    @ifnotempty{#3}{ (#3)}%
    hspace{fctaylorlabelsep}%
    phantomsection%%% THIS LINE ADDED
  }%
}
makeatother

newenvironment{fctayloritemize}
 {list{}{%
    leftmargin=fctaylortheoremindent
    labelwidth=dimexprfctaylortheoremindent-labelseprelax
    itemindent=0pt
  }}
 {endlist}

NewDocumentCommand{newfctaylortheorem}{smomo}{%
  IfBooleanTF{#1}
   {newtheorem*{fctaylor@#2}{#4}}
   {IfNoValueTF{#3}
     {IfNoValueTF{#5}
       {newtheorem{fctaylor@#2}{#4}}
       {newtheorem{fctaylor@#2}{#4}[#5]}}
     {newtheorem{fctaylor@#2}[fctaylor@#3]{#4}}}%
  NewDocumentEnvironment{#2}{o}
   {IfNoValueTF{##1}{begin{fctaylor@#2}}{begin{fctaylor@#2}[##1]}%
    begin{fctayloritemize}item[thisthmheadhfill]}
   {end{fctayloritemize}end{fctaylor@#2}}%
}

theoremstyle{fctaylor}
newfctaylortheorem{mytheorem}{Theorem}[chapter]
newcommand{thistheoremname}{}
newfctaylortheorem{genericthm}[mytheorem]{thistheoremname}
newenvironment{custom}[1][Custom]
  {renewcommand{thistheoremname}{#1}%
   begin{genericthm}}
  {end{genericthm}}
newfctaylortheorem*{genericthm*}{thistheoremname}
newenvironment{custom*}[1][Custom]
  {renewcommand{thistheoremname}{#1}%
   begin{genericthm*}}
  {end{genericthm*}}

begin{document}
section{One}

begin{custom}{Custom Title}[optional words]label{A}
lipsum*[2]
end{custom}

newpage

begin{custom}label{B}
Even though I've been teaching for n! years, every class is a fresh
adventure -- An adventure. That word is a euphemism for the stark reality,
which is that every class contains some totally shocking development
that I have never seen before and haven't any idea how to cope with.

Just last year, for the first time in many years, I taught a section
of the junior-level mathematical analysis course. You know --  the one
where students meet proofs in analysis for the first time. The one where
students and epsilons meet, eyeball to eyeball, and it isn't the epsilons
that blink. The one where students decide that they really wanted to be
doctors and lawyers after all.
end{custom}

newpage

Here are the references: ref{A} and ref{B}.

end{document}

begin{custom}{Custom Title}[optional words]label{A}
lipsum*[2]
end{custom}

begin{custom}label{B}
...
end{custom}