How get the values from combobox to the main file to executed some function UiPath Custom Activity

Question

I am trying to create a custom activity in the visual studio for UiPath.

My visual studio code contains an ActivityDesigner for the interface. I have created that, but I cannot get value from the Combobox to the main file where the execution function is written.

ActivityDesigner.xaml

    <sap:ActivityDesigner x:Class="AchalSharma.ActivityDesigner1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sap="clr-namespace:System.Activities.Presentation;assembly=System.Activities.Presentation"
    xmlns:sapc="clr-namespace:System.Activities.Presentation.Converters;assembly=System.Activities.Presentation"
    xmlns:sapv="clr-namespace:System.Activities.Presentation.View;assembly=System.Activities.Presentation" 
    xmlns:c="clr-namespace:AchalSharma">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width = "Auto" />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>

        <Grid.RowDefinitions>
            <RowDefinition Height = "*" />
            <RowDefinition Height = "*" />
            <RowDefinition Height = "*" />
        </Grid.RowDefinitions>

        <TextBlock Grid.Row = "0" Grid.Column = "0" Text = "Retrieve" Margin = "5" HorizontalAlignment = "Left" VerticalAlignment = "Center" Width = "70" />
        <ComboBox Grid.Row = "0" Grid.Column = "1" Margin = "5" Name="Retrieve" HorizontalAlignment="Left" VerticalAlignment="Top" Width="153">
            <ComboBoxItem Content="Current Date and Time" IsSelected="True"></ComboBoxItem>
            <ComboBoxItem Content="Current Date"></ComboBoxItem>
        </ComboBox>

        <TextBlock Grid.Row = "1" Grid.Column = "0" Text = "Time Zone" Margin = "5" HorizontalAlignment = "Left" VerticalAlignment = "Center" Width = "70"/>
        <ComboBox Grid.Row = "1" Grid.Column = "1" Margin = "5" Name="TimeZone" HorizontalAlignment="Left" VerticalAlignment="Top" Width="153">
            <ComboBoxItem Content="System Time Zone" IsSelected="True"></ComboBoxItem>
            <ComboBoxItem Content="Specific Time Zone"></ComboBoxItem>
        </ComboBox>
    </Grid>
</sap:ActivityDesigner>

Class1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Activities;
using System.ComponentModel;

namespace AchalSharma.DateAndTime
{
    public class GetCurrentDateAndTime : CodeActivity
    {
    
        [Category("Input")]
        //I want here input from Combobox the user will select.

        [Category("Output")]
        public OutArgument<String> CurrentDateAndTime { get; set; }
        protected override void Execute(CodeActivityContext context)
        {
            if (Retrieve == Retrieves.CurrentDateAndTime)
            {
                String date = DateTime.Now.ToString();
                CurrentDateAndTime.Set(context, date);
            }
            else
            {
                String date = DateTime.Now.ToString("MM/dd/yyyy");
                CurrentDateAndTime.Set(context, date);
            }
        }
    }
}

Help in getting the selected value from Combobox to the main file for execution.