你可以像下面这样在val1属性中设置字符串格式，然后进行绑定。

public string _val1;
    public string val1
    {
        get
        {
            return string.Format("Hello, {0}", _val1);
        }
        set
        {
            _val1 = value;
        }
    }

更新：

Xaml：

<local:CommaIntLbl TextVal="{Binding val1}" />

自定义控件：

public class CommaIntLbl : Label
{
    public CommaIntLbl() : base()
    { }
    private string _text;
    public string TextVal
    {
        get { return _text; }
        set
        {
            _text = value;
            OnPropertyChanged();
        }
    }
    public static readonly BindableProperty TextValProperty = BindableProperty.Create(
             nameof(TextVal),
             typeof(string),
             typeof(CommaIntLbl),
             string.Empty,
             propertyChanged: (bindable, oldValue, newValue) =>
             {
                 var control = bindable as CommaIntLbl;
                 control.Text = String.Format("Hello, {0}", newValue);
             });
}

代码隐藏：

public string _val1;
    public string val1 { get; set; }

    public Page19()
    {
        InitializeComponent();
        val1 = "1234";
        this.BindingContext = this;
    }