How to replace the number of letters of a word with a character?

There is a ready builder to do this so it is very simple.

using System;

public class Program {
    public static void Main() {
        var texto = "carambola";
        var adivinha = new String('_', texto.Length);
        Console.WriteLine(adivinha);
    }
}

See running on .NET Fiddle . And at Coding Ground . Also put it on GitHub for future reference .

If you make a loop by adding strings is slower and does damage to the garbage collector . It may not be important to do it in a very simple game, but if you learn wrong something simple will reproduce something that matters more. At least you could use StringBuilder in this case , but it would also be an exaggeration.

You can use the string.Length to know the size of the word you have. Then you can Iterar over that size by typing "_"

Exemplary:

for(int i=0; i< string.Legth; i++)
{
  // aqui você pode criar um string igual ao seu exemplo
  novaString += "_";
}

Or with foreach

foreach(Char caracter in SuaString)
{
  // aqui você pode criar um string igual ao seu exemplo
  novaString += "_";
}

Then you can use your novaString to show the example screen, there are other ways to do it depends on how you will show this text.

In addition to what has been previously answered you can use regular expression 'Regex' with the replace function, replacing the characters.

string txt = "Primeiro Teste";
string forca=   Regex.Replace(txt, "[\w]", "_");

O resultado é: ________ _____

If you prefer to make similar to the gallows just place a space after the Undescore character:

string txt = "Primeiro Teste";
string teste2 = Regex.Replace(txt, "[\w]", "_ ");

O resultado é: _ _ _ _ _ _ _ _  _ _ _ _ _