I don't get it..

What code am i supposed to write down to add a keyframe with name/value combination to the effect ?

Look to the title of your post. It asks not for a script but for changing the (default) keyframe settings of an OFX, that is how I read your question and that has nothing to do with a script, unless you are using a script to make keyframes. But than, what kind of script are you talking about?

What I'm trying to say to make a preset means that in an effect you can make settings plus keyframe them and than you can make of that changed OFX a preset by giving it a name and store that through the map symbol , after you gave it your own name. Next time you open that OFX you can use yours.

Hey, i've meant the OFX parameter keyframe in the C# scripting api.. I put this in the category 'Scripting' ..

I've been writing a plug-in for Vegas Pro in that language, i search for a function that would allow me to Add / Change keyframes at specific position..

So let's say, for that image you posted..

Program.cs

Effects("AddNoise").KeyFrames("NoiseLevel").SetValue(0,1);

This adds a keyframe at Frame 0 (beggining) of that effect at value 1 ..

Effects("AddNoise").KeyFrames("NoiseLevel").SetValue(30,0)

This adds a keyframe at Frame 30 (second 1) of that effect at value 0...

So the final effect would be the noise getting lower to complete abscence at the Frame 30..

Do you wonder, Why i want to program this ?

Well, i'm trying to make an effect randomizer, that means, that it would create random keyframes with random values, because chaos fascinates me to a degree.. I can make that manually for few effects, but i wonder, how would it look like with 5 effects ? Or 20 effects ? As many as can my CPU handle in the rendering process..

I also want to make a noise effect, which would add tiny random values to the keyframes and their values, so i can see if my editing is precise by seeing how would it look like if it was a bit different..

I think that the bigger negative impact would the small changes make on the perceived quality, the more memorable would the original composition be.. If the changes would be positive, it would show me that i don't pay enough attention to specific effects to that extent..

Not sure which type of variable you're looking at changing but this will give you the general rule-of-thumb on how to add keyframes to effect:

OFXDouble2DParameter p2 = (OFXDouble2DParameter)ofx[Parameter];
  //Find the parameter to change
p2.IsAnimated = true;
  //If you're keyframing, this MUST be true

Timecode tc = //Timecode for the keyframe

OFXDouble2D oc;
oc.X = //x value
oc.Y = //y value

p2.SetValueAtTime(tc, oc);
 //Add a new keyframe at the tc timecode with the oc value

It does not matter what type of parameter it is, the above process will work to add keyframes.