Microsoft.VisualBasic.dll has an InputBox method which you can use from C# to get a single string.
For example (Add a reference to Microsoft.VisualBasic.dll first)
using Microsoft.VisualBasic;
string response = Interaction.InputBox("Enter a string:", "MyApp", "DefaultString", 0, 0);
Othewise, you'll have to make your own form.
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