[Tex/LaTex] get a ‘macro parameter # in vertical mode’ error

Question

I am trying to define a new command in the following manner:

\newcommand{\textbelow}[2]{\underbrace{#1}_\substack{#2}}

It looks like the underscore character _ is creating the problem. If I remove it, I do not get any error, but that is not what I want to do.

The syntax is correct, because outside of newcommand it works fine.

\[\underbrace{Look Down}_{\substack{hello \\ world}}\]

Why am I not able to make a new command out of this? If it is not possible to write an underscore _ inside newcommand, what is the workaround?

Thank you for your assistance.

---

If you so require, here is a full example of what I am trying to do.

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}

\newcommand{\textbelow}[2]{\underbrace{#1}_\substack{#2}}
\newcommand{\test}[2]{\underbrace{#1}\substack{#2}}

\begin{document}

This \(\underbrace{Look Down}_{\substack{hello \\ world}}\) works fine.

But the newcommand does not:
    \[ \textbelow{Look Down}{hello \\ world} \]

\end{document}

---

Edit:

@egreg's solution does not work for me. Here is a screenshot of the error I get in TeXstudio.

Answer 1

The errors I get are

! Missing { inserted.
<to be read again> 
                   \vcenter 
l.12     \[ \textbelow{Look Down}{hello \\ world}
                                                  \]
? 
! Missing } inserted.
<inserted text> 
                }
l.12 ...[ \textbelow{Look Down}{hello \\ world} \]

which clearly stem from the missing braces around \substack{...}.

Indeed, if I add the braces, the example code runs without problems.

\documentclass{article}
\usepackage{amsmath}

\newcommand{\textbelow}[2]{\underbrace{#1}_{\substack{#2}}}

\begin{document}

This \(\underbrace{Look Down}_{\substack{hello \\ world}}\) works fine.

But the newcommand does not:
    \[ \textbelow{Look Down}{hello \\ world} \]

\end{document}

However, this doesn't seem good for typesetting text.