How to convert a string to securestring explicitly

The simplest approach is to iterate over the source string and append one character at a time to the secure string, like so:

var secure = new SecureString();
foreach (char c in textbox1.Text)
{
    secure.AppendChar(c);
}

Invent once and reuse lots. Create a simple extension method to extend the string base class and store it some static utilities class somewhere

using System.Security;

/// <summary>
/// Returns a Secure string from the source string
/// </summary>
/// <param name="Source"></param>
/// <returns></returns>
public static SecureString ToSecureString(this string source)
{
    if (string.IsNullOrWhiteSpace(source))
        return null;
    else
    {
        SecureString result = new SecureString();
        foreach (char c in source.ToCharArray())
            result.AppendChar(c);
        return result;
    }
}

and then call as follows:

textbox1.Text.ToSecureString();