If you want to to use the indexing of the shares with Directa Sim and you are not a Directa customer yet, you must open a Directa account.
You can open the Directa account directly online, you just need to log in, access your Dashboard (by clicking the icon in the top right), Account, Indexing and click on the appropriate link.
You will be than redirected to a new page, where through a form you can enter all the data necessary for opening your Directa account.
A few days after completing the form, you will be contacted by Directa for the verification activity for the purposes of anti-money laundering legislation.
After this verification, you will receive your Directa account number and your 5-digit customer number.
Now you just need to enter your Directa customer number as explained here.