How does one indicate the corresponding author when using the authblk package?

This is more a workaround than an official solution I guess. But depending on the journal's author-guidelines for your manuscript this formatting might look more appropriate. It includes the indicator for the corresponding author as an affiliation.

Example code:

\listfiles
\documentclass[titlepage]{article}
\usepackage[affil-it]{authblk}
\usepackage[english]{babel}
\usepackage{blindtext}

\title{An efficient method for exploiting midichlorians in weak life-forms}

\author[1,*]{Darth Vader}
\author[2]{Darth Sidious}

\affil[1]{Office of the Supreme Commander of the Imperial Forces, The Galactic Empire, The Bridge, Executor}
\affil[2]{Order of the Sith Lords, LiMerge Power Building, The Works, Coruscant}
\affil[*]{Corresponding author: Darth Vader, [email protected]}

\begin{document}

\maketitle
\blindtext
\end{document}

Example picture:

You can do it using \thanks , but if you are using the titlepage option (as in \documentclass[titlepage]{article}) then you should include the line \renewcommand{\thefootnote}{\fnsymbol{footnote}} in the header to make the footnote marker be an asterisk. We can also place the line \renewcommand{\thefootnote}{\arabic{footnote}} after \maketitle, to undo the global change. Here is a MWE:

\documentclass[titlepage]{article}
\usepackage[affil-it]{authblk}
\usepackage[english]{babel}
\usepackage{blindtext}

\renewcommand{\thefootnote}{\fnsymbol{footnote}}
\title{An efficient method for exploiting midichlorians in weak life-forms}

\author[1]{Darth Vader \thanks{Corresponding author}}
\author[2]{Darth Sidious}

\affil[1]{%
  Office of the Supreme Commander of the Imperial Forces, 
  The Galactic Empire, The Bridge, Executor}
\affil[2]{%
  Order of the Sith Lords, LiMerge Power Building, The Works, Coruscant}

\begin{document}

\maketitle
\renewcommand{\thefootnote}{\arabic{footnote}}
\blindtext
\end{document}

I found this question when googling for setting corresponding authors in ACM templates. This is how to set corresponding authors in an ACM template:

\author{Author 1}
\affiliation{\institution{Affiliation}}
\email{email}
\authornote{Corresponding authors}

\author{Author 2}
\affiliation{\institution{Affiliation}}
\email{email}
\authornotemark[1]

\author{Author 3}
\affiliation{\institution{Affiliation}}
\email{email}

Here, author 1 and 2 are both set as corresponding authors, but author 3 isn't. Further explanation can be found here: https://www.acm.org/binaries/content/assets/publications/consolidated-tex-template/acmart.pdf