Custom LaTeX macros and newcommands

Yes, you can specify your favorite macros and add your favorite packages (this is a list of LaTeX packages we support)

In Edit mode, click on Edit Macros in the toolbar, under Settings ⚙️
Then, in header.tex you can define your macros and packages.

Macros
For example, if you put this snippet in your header.tex:

\newcommand{\msun}{\,\mathrm{M}_\odot}
and then go back to your article and write in a LaTeX block Hello $\msun$, you will generate “Hello M⊙”.

Packages
For example, if you put this snippet in your header.tex:
\usepackage{siunitx}
and then go back to the Main view and in a LaTeX block you can now type \si{\meter} which will render as m.

Export style specific commands
In the header.tex you can also specify some commands which are style specific and only modify the exported document. For example, if your export style file asks for a corresponding address in the form \corraddress{XXX}, you can include in the header.tex (in the right place --- look for the instructions), the following snippet which will produce a corresponding address box:
\corraddress{Author One PhD, Department, Institution, City, State or Province, Postal Code, Country}

Read more: Can I use a custom LaTeX class file?