Here are some instructions for enabling RLV in various viewers.

Firestorm

• Avatar / preferences / firestorm / general
• Put a tick in the box called “Allow Remote Scripted Viewer Controls [RLVa]”
• Click apply.
• Relog.

Singularity
Edit preferences (ctrl-p).  In the Preferences dialog, select the System tab then Security & Privacy (along the top).  Near the bottom you will find a checkbox for Restrained Love API Support (RLVa)  Make sure it is checked.  Press OK or Apply. Relog for the changes to take effect.

Imprudance

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Enable Advanced Menu if not enabled (ctrl+alt+shit+d)
Click in Restrained Love Support in the advanced menu
relog.

Restrained Love Viewer

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yet to test this, there is no linux version so have not checked yet.