https://tex.stackexchange.com/q/587856/35864

textciteshorthandintrocomp.tex

\documentclass[british,a4paper]{article}
\usepackage{babel}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{csquotes}
\usepackage[backend=biber, style=ext-authoryear-comp]{biblatex}
\DefineBibliographyStrings{english}{citedas = {hereafter}}

\begin{filecontents*}[force]{\jobname.bib}
@article{jd14,
  author  = {Doe, J. and Smith, J. and Bar, F.},
  title   = {Some Title},
  journal = {Some Journal},
  year    = {2014},
  shorthand = {JD14},
}
@article{jd15,
  author  = {Doe, J. and Smith, J. and Bar, F.},
  title   = {Some Title},
  journal = {Some Journal},
  year    = {2015},
  shorthand = {JD15},
}
@article{jd13,
  author  = {Doe, J. and Smith, J. and Bar, F.},
  title   = {No shorthand here},
  journal = {Some Other Journal},
  year    = {2013},
}
@article{b12,
  author  = {Bar, F.},
  title   = {Also No shorthand},
  journal = {Yet Another Journal},
  year    = {2012},
}
\end{filecontents*}
\addbibresource{\jobname.bib}

\begin{document}
  \textcite{jd14} again \textcite{jd14}.

  \textcite[see][45]{jd15} again \textcite[see][45]{jd15}.

  \textcite{jd14,jd15} again \textcite{jd14,jd15}.

  \textcite{jd13,jd15} again \textcite{jd13,jd15}.

  \textcite{b12,jd14} again \textcite{b12,jd14}.

  Just for comparison \textcite{jd13}. But alas \textcite{b12,jd13} doesn't work

  \textcite[34-36]{jd13} again \textcite[see][34-36]{jd13}

  \printbibliography
\end{document}