Apostrophe s when citing

Question

I cite elements from my bibfile using \citet{key}. The reference appears as "Name1, and Name2 (year)" in text where year is a link leading to the bibliography. Now, I want to include an 's after the authors name, e.g.

Using Name1, and Name2's (2019) formula, I compute ...

Any ideas of I could adjust the \citet command?

---

I use biblatex as follows.

\usepackage[style=ext-authoryear-comp,
            backend=biber,
            natbib=true,
            uniquename=false,
            uniquelist=false,
            giveninits=false,
            dashed=false,
            backref=false,
            maxbibnames=99,
            maxcitenames=3,
            isbn=false,
            doi=false]{biblatex}

Answer 1

A cheap way to produce citations like these is

\citeauthor{sigfridsson}'s \parencite*{sigfridsson}

this should work out of the box for most styles.

If you'd like to see a solution with a dedicated command, we can modernise Audrey's answer to Author name of \textcite as possessive to create a new \posscite command that automatically inserts "'s" at the end of the names. Again, this solution is pretty style independent.

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[style=ext-authoryear-comp, backend=biber,]{biblatex}

\DeclareNameWrapperFormat{labelname:poss}{#1's}

\newrobustcmd*{\posscitealias}{%
  \AtNextCite{%
    \DeclareNameWrapperAlias{labelname}{labelname:poss}}}

\newrobustcmd*{\posscite}{%
  \posscitealias
  \textcite}

\newrobustcmd*{\Posscite}{\bibsentence\posscite}

\newrobustcmd*{\posscites}{%
  \posscitealias
  \textcites}

\addbibresource{biblatex-examples.bib}

\begin{document}
\posscite{sigfridsson}
\printbibliography
\end{document}

Several alternatives where you give the possessive marker manually are discussed in Adding a third option for inserting possessive to \textcite. If you are interested in one of those solutions, they could be modified for your use case as well.

---

If you want to attach the "'s" to the name at all times and not the "et al." the code gets a bit longer, because we can no longer use name wrappers to inject the "'s".

\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{csquotes}

\usepackage[style=ext-authoryear-comp, backend=biber,]{biblatex}

\DeclareNameFormat{labelname:poss}{%
  \ifcase\value{uniquename}%
    \usebibmacro{name:family}
      {\namepartfamily}
      {\namepartgiven}
      {\namepartprefix}
      {\namepartsuffix}%
  \or
    \ifuseprefix
      {\usebibmacro{name:given-family}
        {\namepartfamily}
        {\namepartgiveni}
        {\namepartprefix}
        {\namepartsuffixi}}
      {\usebibmacro{name:given-family}
        {\namepartfamily}
        {\namepartgiveni}
        {\namepartprefixi}
        {\namepartsuffixi}}%
  \or
    \usebibmacro{name:given-family}
      {\namepartfamily}
      {\namepartgiven}
      {\namepartprefix}
      {\namepartsuffix}%
  \fi
  \ifnumequal{\value{listcount}}{\value{liststop}}
    {'s}
    {}%
  \usebibmacro{name:andothers}}

\newrobustcmd*{\posscitealias}{%
  \AtNextCite{%
    \DeclareNameAlias{labelname}{labelname:poss}}}

\newrobustcmd*{\posscite}{%
  \posscitealias
  \textcite}

\newrobustcmd*{\Posscite}{\bibsentence\posscite}

\newrobustcmd*{\posscites}{%
  \posscitealias
  \textcites}

\addbibresource{biblatex-examples.bib}

\begin{document}
\posscite{sigfridsson}

\posscite{aksin}

\printbibliography
\end{document}