Unreal Engine & Houdini How can I show Unreal where to Find Houdini Engine?

The problem I am having is, I'm trying to install the Houdini Engine Plugin into Unreal but Unreal doesn't know how to find Houdini. I looked through all the Unreal and Houdini Forums but they all point to Windows or MacOS solutions. This is the error I am getting.

Generating Make project files: Generating data for project indexing...
Houdini Engine : Houdini 19.5.605 could not be found. Houdini Engine will not be available in this build.

I keep finding the same solution "set the $HFS environment variable to your houdini install folder" but where is this ie.. /path/to/file/or/folder? The Install is /opt/hfs19.5.605 The symlink points to /home/houdini19.5 In the HoudiniEngine.Build.cs I find these entries:

// Find HFS { string HFSPath = GetHFSPath(); HFSPath = HFSPath.Replace("\", "/");

    if( HFSPath != "" )
    {

and

private string GetHFSPath() { string HoudiniVersion = "19.5.605"; bool bIsRelease = true; string HFSPath = ""; string RegistryPath = "HKEY_LOCAL_MACHINE\SOFTWARE\Side Effects Software"; string Registry32Path = "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Side Effects Software"; string log;

    if ( !bIsRelease )
    {
        // Only use the preset build folder
        Log.TraceVerbose("Using stamped HFSPath:" + HFSPath);
        return HFSPath;
    }

    // Look for the Houdini install folder for this platform
    PlatformID buildPlatformId = Environment.OSVersion.Platform;
    if (buildPlatformId == PlatformID.Win32NT)
    {
        // Look for the HEngine install path in the registry
        string HEngineRegistry = RegistryPath + string.Format(@"\Houdini Engine {0}", HoudiniVersion);
        string HPath = Microsoft.Win32.Registry.GetValue(HEngineRegistry, "InstallPath", null) as string;
        if ( HPath != null )
        {
            log = string.Format("Houdini Engine : Looking for    Houdini Engine {0} in {1}", HoudiniVersion, HPath );
            Log.TraceVerbose( log );
            if ( Directory.Exists( HPath ) )
                return HPath;
        }

But when I add /opt/hfs19.5 or hfs19.5.605 in the HFSPath lines noting happens.