To add effect TO any VST preset, you do exactly as with native LMMS-instruments: Add the effect on the FX-tab
But i think you ask how to add VST-effects to LMMS ?
Here you should read: https://lmms.io/wiki/index.php?title=Wo ... Ts_effects
To add effect TO any VST preset, you do exactly as with native LMMS-instruments: Add the effect on the FX-tab