First name will be shown as an initial on the MFD and in the standings, but will be shown in full in the pit stop menus.
Last name is always shown in full.
"Short name", or nickname is only visible to your teammates in the MFD.
If you'd like to use a nickname rather than your actual name, leave the first name field blank and fill in your nickname as both the last name and short name.
Please provide your steamID64 (Community ID), grabbed from this website:
https://steamid.uk/