要将AdMob关闭按钮放置在iPhone 12的安全区域上方，可以使用以下代码示例：

Objective-C:

// 获取安全区域Insets
UIEdgeInsets safeAreaInsets = UIApplication.sharedApplication.windows.firstObject.safeAreaInsets;

// 创建关闭按钮
UIButton *closeButton = [UIButton buttonWithType:UIButtonTypeCustom];
[closeButton setTitle:@"关闭广告" forState:UIControlStateNormal];
[closeButton setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
[closeButton addTarget:self action:@selector(closeAd) forControlEvents:UIControlEventTouchUpInside];

// 设置关闭按钮的位置和大小
CGFloat buttonHeight = 50.0;
CGFloat buttonWidth = 100.0;
closeButton.frame = CGRectMake(safeAreaInsets.left, safeAreaInsets.top, buttonWidth, buttonHeight);

// 将关闭按钮添加到广告视图中
[self.view addSubview:closeButton];

Swift:

// 获取安全区域Insets
if let safeAreaInsets = UIApplication.shared.windows.first?.safeAreaInsets {

    // 创建关闭按钮
    let closeButton = UIButton(type: .custom)
    closeButton.setTitle("关闭广告", for: .normal)
    closeButton.setTitleColor(.black, for: .normal)
    closeButton.addTarget(self, action: #selector(closeAd), for: .touchUpInside)

    // 设置关闭按钮的位置和大小
    let buttonHeight: CGFloat = 50.0
    let buttonWidth: CGFloat = 100.0
    closeButton.frame = CGRect(x: safeAreaInsets.left, y: safeAreaInsets.top, width: buttonWidth, height: buttonHeight)

    // 将关闭按钮添加到广告视图中
    self.view.addSubview(closeButton)
}

这段代码首先获取了iPhone 12的安全区域Insets，然后创建了一个UIButton作为关闭按钮，设置了按钮的位置和大小，最后将按钮添加到广告视图中。你可以根据需要调整按钮的样式和位置。