What do I have to tell the IT system house?

The best way to do this is to send an email.

Here is a short email template:

… we would like to use eSignatures from tegoly in the future. We hereby order DATEVconnect (item no. 95280) and DATEVconnect document management, item no. 63218.

Please contact tegoly (contact@tegoly.com) for further steps and make an appointment to install tegolySIGN.