How do I reference a file id from another Wix fragment?

How do I reference a file id from another Wix fragment?

I'm working on a Wix 3.6 installer. I've grouped all of my features into
different fragments (files). Some fragments are part of a another project
(WixLib). What I need to do is get the path of a file from another
fragment. For some reason this doesn't work. The variable is always empty.
For example, this is one of the fragments referenced from another Wix
project:
<Fragment>
<DirectoryRef Id="INSTALLLOCATION">
<Directory Id="ScriptsDir" Name="Scripts" FileSource="...">
<Component Id="ScriptsComponent" Guid="...">
<File Id="CTTEXE" Name="ctt.exe"/>
<File Name="ctt.exe.config"/>
</Component>
</Directory>
</DirectoryRef>
</Fragment>
This is how I try to access the variable:
[#CTTEXE]
I also tried referencing it as a property with PropertyRef but I get an
unresolved reference error.
What am I missing?