From the website:

From website by going to the Organisations section:

Search for the desired organisation by clicking on the search bar, either from the different categories that appear (Purchases, Family, Finance, etc.), or by entering the name of the organisation directly.

Then click on the organisation's logo and fill in your account details for that organisation.

If you receive a security code by text message, enter it and wait a few seconds to confirm the subscription.

After confirming your account details, you will start to automatically receive your documents in Digiposte.

From the mobile app:

Go to Organisations, which can be accessed via the menu at the bottom of your screen.

To add an organisation, tap the + button at the bottom right. You can also add an organisation from the various categories. Then enter your account details for the organisation.

After confirming your account details, you will start to automatically receive your documents in Digiposte.