I have in mind this.
First time that the form is opened there's a button to click that will ask for username ans password and will store it.
Next time I'll open the Form and click the button it automatically login (the exe is intended for a user only so there's no need to ask user and psw again).
The exe should contain the key to decrypt the string stored.
OK, but never UNDER ANY CIRCUMSTANCES store and use the passphrase as entered because of the risk that the lackwitted user will use the same passphrase for more than one service. Use a seeded hash on the password, if necessary truncate the result (decent hash algorithms are supposed to obfuscate bit patterns adequately), and use /that/ as the password being submitted to the remote service.
What you do locally is your responsibility: it's your computer, or you've got a professional relationship with your clients and are protected by your liability insurance. But you must ALWAYS assume that the service to which you're connecting will eventually get hacked, and you should ALWAYS assume that they aren't protecting their users' passwords adequately, hence you should NEVER put yourself in the position of passing on an incautiously-selected passphrase without hashing it.
I must admit that I do have reservations about your writing code from scratch to store passphrases locally, and if I were doing it I'd be very much inclined to research the availability of some existing library complying with PCI DSS, or interworking with one of the storage mechanisms used by various browsers etc.
(Noting Thaddy's post which has beaten me to the post, but thinking that we're in broad agreement.)
(And noting GetMem's which arrived while I was typing that...)
MarkMLl