更改现有的iframe源:

Change the existing iframe source:

function changeIframe(url) {
  document.getElementById('change').src = url;
}

使用onchange事件而不是onsubmit:

<input type="text" id="minecraftInput" onchange="changeIframe(this.value)" id="detroit" placeholder="https://classic.minecraft.net/?join=somelettersand#s" name="link" required>