Restrict assigned to field

How can i have the assigned to field only populate agents and not pull all active 0365 users in the tenant?

Hello! Please open Settings wheel > Site permissions > Advanced permissions settings:

Create a new group and call it, for example, "HelpDesk agents":


Add to the group the SharePoint users that are considered as agents:


Go to the "Tickets" list > List settings > Assigned to column:

And restrict the available users by the "HelpDesk agents" group: