How to assign a value to BSTR in C#

As part of a legacy automation, we try to assign "c:\temp" to a COM object's property.

var host = engine.Session.Hosts.Item[HostName]; 
host.TemporaryDirectory = TemporaryDirectoryPath;

During the run, we saw a COMException - "Type mismatch...0x80020005..."

It seems that the property is in typeof BSTR and have a default value "50".

Can you please advice how can I perform a string assignment in this case?