var entry = new Entry { Placeholder = "Enter text here" };

entry.Keyboard = Keyboard.Create(KeyboardFlags.Suggestions | KeyboardFlags.CapitalizeCharacter);

<Entry Placeholder="Enter text here">

    <Entry.Keyboard>

        <Keyboard x:FactoryMethod="Create">

            <x:Arguments>

                <KeyboardFlags>Suggestions,CapitalizeCharacter</KeyboardFlags>

            </x:Arguments>

        </Keyboard>

    </Entry.Keyboard>

</Entry>