How can I assign all incoming tickets to a particular agent on the portal?

This can be done by creating a new Workflow Automator rule (Admin > Helpdesk Productivity > Workflow Automator> New Automator) in Freshservice.

Scenario A: Assign all tickets by default to an agent

For assigning all the tickets to one particular agent, the condition of the Workflow Automator rule has to be set at Null (set at 'Select Condition'), in which case, the Workflow Automator would work for all incoming tickets.

Scenario B: For assigning specific tickets to one particular agent, apply the desired Conditions (can be based on either ticket, contact and company fields) and set up the required Action block.