You can find the Commissions Agency in Aht Urhgan Whitegate K-9.
Speak with Rytaal in order to obtain your Imperial Army I.D. Tag (which is required to participate). When you start out with Assault you will be given 3 Imperial Army I.D. Tags - you use one for each Assault Mission you do. One Imperial Army I.D. Tag is restored every 24 hours from when you took your first one, until you have all 3 Imperial Army I.D. Tags back. It is possible to have a fourth Imperial Army I.D. Tag, by choosing to take a Imperial Army I.D. Tag when you have 3 already available, and waiting 24 hours for a new Imperial Army I.D. Tag to be offered.
You will also find the five Assault Mission Giver NPCs: Yahsra for Leujaoam Sanctum, Isdebaaq for Mamool Ja Training Grounds, Famad for Lebros Cavern, Lageegee for Periqia and Bhoy Yhupplo for Ilrusi Atoll. They will tell you how many Assault Points you currently have for each Assault and can exchange them into a reward. After obtaining an Imperial Army I.D. Tag, talk to one of them to select an Assault.