I am using an Entry (CustomEntry with no underline) like below:

Code:

<renderer:CustomEntry
    x:Name="search_entry"
    Placeholder="Search by request id, requester's name, title, or description"
    Style="{StaticResource SearchEntryStyle}"/>

Screenshot:

The placeholder text is lengthy, so it's not possible to read the complete text of it. Is there any way to add an auto scroll for the Entry placeholder?

I don't think there is an easy way to do this using the Entry control itself, e.g. with a custom renderer.

However, perhaps you can fake it by binding the Placeholder to a property in the code-behind or ViewModel and then update the text and make it look like it scrolls by removing the characters at the beginning one-by-one.

Here's a sample implementation with a timer which updates the placeholder every 200 milliseconds, but you can of course also do it differently:

Xamarin.Forms

public class SomeViewModel : INotifyPropertyChanged
{
    private const string Placeholder = "This is a very long text for a placeholder of an Entry that seems to be animated... ";

    private string _placeholderText = Placeholder;
    public string PlaceholderText
    {
        get => _placeholderText;
        set
        {
            if(_placeholderText == value) return;
            _placeholderText = value;
            OnPropertyChanged();
        }
    }

    public SomeViewModel()
    {
        Device.StartTimer(TimeSpan.FromMilliseconds(200), () =>
        {
            var firstChar = PlaceholderText[0];
            var sb = new StringBuilder(PlaceholderText);
            sb.Remove(0, 1);
            sb.Append(firstChar);
            PlaceholderText = sb.ToString();

            return true;
        });        
    }

    //...
}

.NET MAUI

In .NET MAUI, the implementation is essentially the same, except that you'll need to use a DispatcherTimer instead of the deprecated DeviceTimer:

public SomeViewModel()
{
    var timer = Application.Current.Dispatcher.CreateTimer();
    timer.Interval = TimeSpan.FromMilliseconds(200);
    timer.Tick += (s, e) =>
    {
        var firstChar = PlaceholderText[0];
        var sb = new StringBuilder(PlaceholderText);
        sb.Remove(0, 1);
        sb.Append(firstChar);
        PlaceholderText = sb.ToString();
    };
    timer.Start();
}

Binding the Placeholder

Then in your View, you can bind to the PlaceholderText property:

<renderer:CustomEntry
    x:Name="search_entry"
    Placeholder="{Binding PlaceholderText}"
    Style="{StaticResource SearchEntryStyle}"/>