Thank you for your post!
If using an SSL Certificate, I believe you are looking for Admin Console > Database Server > Security > Use SSL for Progressive Downloading. "Progressive downloading allows clients to use interactive content as it is being downloaded. Restart the FileMaker Server service (Windows) or FileMaker Server background processes (OS X) to apply a change to this setting."
Amazing TSPigeon. Thank you very much for this. I have tested it and it seems to do exactly what I need! Phew