using (MagickImage image = new MagickImage("input.jpg"))
{
image.ContrastStretch(new Percentage(25), new Percentage(75));
// Save image
image.Write("output.jpg");
}
contrast-stretch 25%,75% is not the same as using -level 25%,75% in percent. Contrast-stretch clips by percentage of pixels. Level adjusts by percent graylevel. So you are using the wrong function in Magick.NET
You posted this in the Magick.NET forum, but it looks more like Magick++ to me. I don't use either.
Anyhow, as Fred says the constrastStretch() function will correspond to the CLI command "-contrast-stretch". As you want "-level", the level() function seems more likely.