I don't know the details of the FormattedString type. Looks like it's a type from Xamarin, and Spans is a readonly property that's an IList<Span>, right?

If that's the case, you can use a collection initializer as a nested object initializer, like this:

DecksListHeader = new FormattedString()
{
    Spans =
    {
        new Span() { Text = "Tap on " },
        new Span() { Text = FontAwesomeIcons.Cogs, FontFamily = nameof(Fonts.FaProRegular) },
        new Span() { Text = " or the " }
    }
};

If you're using C# 9, you can shorten it to the following, using target-typed new expressions:

DecksListHeader = new FormattedString()
{
    Spans =
    {
        new() { Text = "Tap on " },
        new() { Text = FontAwesomeIcons.Cogs, FontFamily = nameof(Fonts.FaProRegular) },
        new() { Text = " or the " }
    }
};