The 'Client Preferred Interpreter' typeahead field is on the Interpreters Tab of the requests below the 'List of Assigned Interpreters' box.
When you or your client, if you have given them access, create a job, a preferred interpreter can be immediately offered the job.  This will occur as soon as the job is saved with the Preferred Interpreter field having a name in it.  See below

Note also that the if the box is checked for 'Show on interpreter portal', this job will appear on the preferred interpreter's Open Jobs tab on the Interpreter Portal if you have configured the interpreter to be able to see open jobs and have a view far enough in the future to see this job. See the screen captures below showing configuration settings.  
The first setting is 'Default for show available job on portal'.  This is on Maintain Company Settings on the Job Settings tab.  It needs to be set to Yes.  

The second setting is Administration/Maintain Users for the specific interpreter. 'Preview Days' needs to be set to allow the interpreter to see available jobs in the future.

As long as the above settings will allow the interpreter to see the job on the Open Jobs tab in his portal, it will appear in red and only on this interpreters portal until it is declined.

So, remember:
1.  If an interpreter is added to Preferred Interpreter field, that interpreter will automatically be offered the job.
2.  It will appear on their portal if you have that interpreter configured to see open jobs and have set his preview out far enough to see this job.
3.  The job will remain on this interpreter's portal only until it is declined.  At which time, it will appear on all qualified interpreters' portals (if appropriately configured).