AddExercise.xaml

<Window x:Class="TodoList.AddExercise"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:TodoList"
        xmlns:ViewModel="clr-namespace:TodoList.ViewModels"
        mc:Ignorable="d"
        Title="AddExercise" Height="500" Width="300">
    <Window.DataContext>
        <ViewModel:MainVM></ViewModel:MainVM>
    </Window.DataContext>
    <Grid>
        <StackPanel>
            <Label FontSize="18">Введите заголовок:</Label>
            <TextBox Text="{Binding AddedTask.Header, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" FontSize="15"></TextBox>
            <Label FontSize="18">Введите основной текст:</Label>
            <TextBox Text="{Binding AddedTask.MainText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" FontSize="15" Height="300" TextWrapping="Wrap"></TextBox>
            <Button Command="{Binding CloseAddWindow, UpdateSourceTrigger=PropertyChanged}" FontSize="18">Добавить</Button>
        </StackPanel>
    </Grid>
</Window>