As soon as you are injected, youll be able to directly execute scripts in the Roblox lua environment

Execute

Put this code in your Execute_Click() event.
private async void execute_Click(object sender, RoutedEventArgs e)
{
    Api.execute("print('Hello, World')"); 
}
Replace print('Hello, World') with the actual Lua code.
Usually, people want their executor to run custom scripts, not pre-set ones, but rather text from an editor. Here is an example for richTextBox1
private async void execute_Click(object sender, RoutedEventArgs e)
{
    Api.execute(richTextBox1.Text);
}
this code will use the content of the richTextBox1