Should I/my supervisor contact the External Examiner nominees before submitting the form to Graduate and Postdoctoral Studies?

No. The integrity of the final doctoral examination relies on a scrupulously impartial outside assessment of the dissertation by an expert who is clearly at arm’s length from both the candidate and the supervisor, and who maintains that relationship during the examination period. It is the responsibility of the research supervisor and graduate program to nominate qualified individuals to serve as External Examiner for your examination; it is the responsibility of Graduate and Postdoctoral Studies to review and approve those nominees, to extend invitations as appropriate, and to deal with all correspondence regarding your external examination. 

The candidate is to have no contact with the External Examiner about the examination until the oral defence is complete. If your supervisor would like to extend an invitation for the External Examiner to attend the oral defence or provide a copy of your abstract, they will be invited to do so once an invitation has been extended by our office. Failure to maintain an arm's length relationship throughout the examination process can result in the disqualification of the External Examiner.

See Selecting the External Examiner for more information.