This can be done by creating a new automation rule under Admin > Automations > Ticket creation tab in Freshdesk.

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

For example:

Condition: Can be left empty

Action:  Assign to Agent > Saul

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.

For example:

Condition: Subject contains 'Article Feedback'

Action: Assign to Agent > ABC

This rule will assign all new incoming tickets which have the subject 'Article Feedback' to the corresponding agent.