It doesn't work on Rider 2018.3.1

[huoguangjin]

Rider 2018.3.1 (Build #RD-183.5253.26, built on December 27, 2018)

[huoguangjin]

Log when trying to toggle highlight:

20:51:54.093 | INFO  | FrontendEditorActionHandler    | Executing backend editor handler 'EditorSelectWord'
20:51:54.093 | INFO  | FrontendActionHandlers         | >> Begin backend 'ExtendSelection' action session
20:51:54.097 | INFO  | FrontendActionHandlers         | << End backend 'ExtendSelection' action session

[huoguangjin]

check the last section about Rider in this sdk doc What is the IntelliJ Platform

Highlights may be implemented by ReSharper

JetBrains Rider uses the IntelliJ Platform differently than other IntelliJ based IDEs. It uses the IntelliJ Platform to provide the user interface for a C# and .NET IDE, with the standard IntelliJ editors, tool windows, debugging experience, and so on. It also integrates into the standard Find Usages and Search Everywhere UI, and makes use of code completion, syntax highlighting, and so on.
However, Rider doesn’t create a full PSI (syntactic and semantic) model for C# files. Instead, it reuses ReSharper to provide language functionality. All of the C# PSI model, all inspections and code rewriting, such as quick fixes and refactorings are run out of the process, in a command- line version of ReSharper. This means that creating a plugin for Rider involves two parts - a plugin that lives in the IntelliJ “front end” to show user interface, and a plugin that lives in the ReSharper “back end” to analyze and work with the C# PSI.
Fortunately, many plugins can simply work with the ReSharper backend. The Rider takes care of displaying the results of inspections and code completion, and many plugins can be written that don’t require an IntelliJ UI component. More details can be found in the Part VIII - Product Specific section.

[huoguangjin]

it seems it's not an easy job to fix this issue

more references:
IntelliJ Platform SDK DevGuide / Rider Plugin Development
Writing plugins for ReSharper and Rider
ReSharper DevGuide / Rider plugins