That's the most basic thing you can do with C#. You could have looked up for some solutions before posting. Anyways try this.
private void button1_Click(object sender, EventArgs e)
{
c.Open();
string w = "insert into checkmultiuser(username) values (@username)";
SqlCommand cmd = new SqlCommand(w, c);
cmd.Parameters.Add("@username";, SqlDbType.VarChar);
cmd.Parameters["@username";].Value = textBox1.Text;
cmd.ExecuteReader();
c.Close();
}