Hi Valeria,
Welcome to the DocuSign Community!
We are glad to have you here and can't wait to help share as much knowledge as possible.
I understand that you are looking to send an envelope to a contact in your Address Book, and I will share the steps to set it up.
You can add an existing contact to your envelope during the “Add Recipients” step of the envelope creation process.
You can find detailed steps on this process, here:
Add Recipients
Regarding the Windows app, there should be no limitation to log into a Free Trial account.
Are you getting an error when logging in?
I would also like to confirm where you are seeing the “I'm the only subscriber?” check?
Feel free to let us know if you need further assistance with this.
Thank you for using DocuSign, we hope you have a wonderful rest of your day!