How to add text to StringFormat ,when the format is taken by a static resource in XAML

I have a xaml like this

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:d="http://xamarin.com/schemas/2014/forms/design" x:Class="eFatura.Tools.StringFormats">
    <ContentPage.Resources>
        <x:String x:Key="formatdatetimelong">ddMMMyy HH:mm:ss.fff tt</x:String>
        <x:String x:Key="formattimewithoutseconds">{0:h:mm tt}</x:String>
        <x:String x:Key="formatdatetimewithoutseconds">{0:M/d/yy h:mm tt}</x:String>
        <x:String x:Key="formatdecimalzeroplaces">{0:0}</x:String>
        <x:String x:Key="formatdecimaloneplaces">{0:0.0}</x:String>
        <x:String x:Key="formatdecimaltwoplaces">{0:0.00}</x:String>
        <x:String x:Key="formatdecimalthreeplaces">{0:0.000}</x:String>
        <x:String x:Key="formatdecimalfourplaces">{0:0.0000}</x:String>
        <x:String x:Key="formatphonenumberusa">{0:(###) ###-####}</x:String>
        <x:String x:Key="formatcurrencyusa">{0:$#,##0.00;($#,##0.00);Zero}</x:String>
        <x:String x:Key="formatpercent">{0:0%}</x:String>
    </ContentPage.Resources>
    <ContentPage.Content>
        <StackLayout>
            <Entry x:Name="_entry2" Text="{Binding Entry2 ,StringFormat={ StaticResource formatdecimalthreeplaces} }" Keyboard="Numeric" TextColor="Black" Placeholder="Db Config" PlaceholderColor="Black" />
            <Entry x:Name="_entry4" Text="{Binding Entry4 ,StringFormat={StaticResource formatdecimalthreeplaces}}" Keyboard="Numeric" TextColor="Black" Placeholder="Db Default" PlaceholderColor="Black" />
            <Entry x:Name="_entry5" Text="{Binding Entry5 ,StringFormat={StaticResource formatdecimaltwoplaces}}" Keyboard="Numeric" TextColor="Black" Placeholder="Db Default" PlaceholderColor="Black" />
            <Entry x:Name="_entry6" Text="{Binding Entry5 ,StringFormat={StaticResource formatdecimaloneplaces}}" Keyboard="Numeric" TextColor="Black" Placeholder="Db Default" PlaceholderColor="Black" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

and the code behind like this

using Xamarin.Forms;
    using Xamarin.Forms.Xaml;
    
    namespace eFatura.Tools
    {
        [XamlCompilation(XamlCompilationOptions.Compile)]
      public class formatedText
        {
            public decimal? Entry1 { get; set; }
            public decimal? Entry2 { get; set; }
            public decimal? Entry3 { get; set; }
            public decimal? Entry4 { get; set; }
            public decimal? Entry5 { get; set; }
        }
        
        
        
        public partial class StringFormats : ContentPage
        {
            private formatedText testFormat = new formatedText()
            {
                Entry1 = new decimal(544616.545546),
                Entry2 = new decimal(544616.545546),
                Entry3 = new decimal(544616.545546),
                Entry4 = new decimal(544616.545546),
                Entry5 = new decimal(544616.545546)
            };
            public StringFormats()
            {
                InitializeComponent();
                this.BindingContext = testFormat;
            }
        }
    }

in XAML I want to make something like:

Text="{Binding Entry5 ,StringFormat='some text { StaticResource formatdecimalthreeplaces}'}"

How can achieve this?

You can create a custom behavior:

public class MyBehavior: Behavior<Entry>
{
    public static readonly BindableProperty CustomTextProperty =
        BindableProperty.Create("CustomText", typeof(string), typeof(MyBehavior), null);

    public string CustomText
    {
        get { return (string)GetValue(CustomTextProperty); }
        set { SetValue(CustomTextProperty, value); }
    }

    protected override void OnAttachedTo(Entry bindable)
    {
        base.OnAttachedTo(bindable);
        bindable.TextChanged += HandleTextChanged;
    }

    private void HandleTextChanged(object sender, TextChangedEventArgs e)
    {
        SetCustomText(sender as Entry);
    }

    protected override void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        base.OnPropertyChanged(propertyName);
    }

    private void SetCustomText(Entry entry)
    {
        entry.Text = $"{CustomText} {entry.Text}";
        entry.TextChanged -= HandleTextChanged;
    }
}

and using it as follow:

<Entry x:Name="_entry2" Text="{Binding Entry2 ,StringFormat={StaticResource formatdecimaltwoplaces}}" Keyboard="Numeric" TextColor="Black" Placeholder="Db Config" PlaceholderColor="Black" >
    <Entry.Behaviors>
        <local:MyBehavior CustomText="Example text"/>
    </Entry.Behaviors>
</Entry>