@Lindon I assume this was the solution?
https://forum.hise.audio//post/89769
Im still a beginner so im not sure.
I did what the post suggested.
I got this now.

Chorale is the name of the my dsp network that was previously in the script fx node.
And i also replace the knob from the interface with a new knob.

However the problem remains, in my daw, when i turn the knob theres no change in gain.
The post also says something about removing something from the interface.js code, im not sure what is meant with that. Im still a beginner and all the example snippets use scriptfx nodes. I can check the forum but half of the answers i do not really understand because i dont understand the program that well. Could you please help me?

I wrapped some simple faust code with HISE.
Heres the faust code:

declare filename "NJERG.dsp";
declare name "NJERG";
import("stdfaust.lib");
gain = hslider("gain", 1, 0, 1, 0.1);
process = _,_ : (*(gain), *(gain));

I added the parameter to the interface and linked it to the faust parameter:

And when i use a wavegenerator in the master chain:

The turning the knob in the interface preview changes the gain audibly.
So i guess all settings are correct.
So then i remove the wavegenerator from the master chain, so it can take input and work as a FX plugin.
I enable vst3 support and then i export it as a VST plugin.
However when i open it in my DAW turning the gain knob does not change the gain audibly. The default value of the gain is 0.5, and i do hear a drop in audio level when opening the plugin so the audio is passing through and multiplied by 0.5. So is there something wrong i did regarding the interface?

EDIT:
I actually didnt select export as FX plugin but as intstrument, that would maybe explain that the gain knob didnt do anything. However now that re-exported and this time i did select FX plugin, it doesnt even pass through the gain anymore: no gain reduction by default value 0.5 and also changing the knob doesnt do anything.