For VST2: Loading presets will have to be done by filling all the parameters. There are about 8000 of them and you'll have to set all of them. (You can also fetch them from Stereo Tool, so the best way is to load the preset in Stereo Tool and then get all the parameter values from it. Certain settings (such as RDS texts) cannot be set in this way and require another calling mechanism.
For VST3: You can send all the settings as a binary blob, I'm not really sure though what the format of this is. (As far as Stereo Tool is concerned it's the full contents of a .sts or .ini file - and you can also read it from Stereo Tool just like with the VST2).
Alternatively, if you only need to talk to Stereo Tool and not to other plugins, we have our own plugin format that makes things a lot easier - it has a "load" function to load settings from a file. You can find the SDK and a whole bunch of binaries (Windows, Linux, Pi, Mac, even iOS) here - the .h file contains some example code: https://www.stereotool.com/download/Ste ... plugin.zip