Frage
Bei getElementById(codeinput) musst du "codeinput" in Anführungszeichen setzen, des Weiteren musst du um den Text des InputFields zu lesen auf die value zugreifen.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Mit Projekt-Key herunterladen | LSprojects</title>
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha1/dist/css/bootstrap.min.css" rel="stylesheet"
integrity="sha384-GLhlTQ8iRABdZLl6O3oVMWSktQOp6b7In1Zl3/Jr59b6EGGoI1aFkw7cmDA6j6gD" crossorigin="anonymous">
</head>
<style>
body {
background-color: rgb(26, 24, 24);
color: white;
}
.btn {
background-color: rgb(161, 156, 156);
color: white;
}
</style>
<script>
function submit() {
let code = document.getElementById("codeinput");
window.location = "/codes/" + code.value + ".html"
}
</script>
<body>
<div class="vertical-center horizontal-center">
<h1>Geben sie den Key hier ein:</h1>
<div class="input-group mb-3 dark-backround">
<button href="submit()" onclick="submit()" class="btn btn-outline-secondary" type="button"
id="load">Laden</button>
<input type="text" class="form-control" placeholder="Projekt-Key" id="codeinput" aria-label=""
aria-describedby="load">
</div>
</div>
<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha1/dist/js/bootstrap.bundle.min.js"
integrity="sha384-w76AqPfDkMBDXo30jS1Sgez6pr3x5MlQ1ZAGC+nuZB+EYdgRZgiwxhTBTkF7CXvN"
crossorigin="anonymous"></script>
</body>
</html>