Foxit Reader can't protect your PDF files with a password. You will need another product, one that's capable of editing various PDF functions and attributes. For this procedure, you can use Foxit PhantomPDF. This is an all around PDF editor that's capable of modifying various PDF settings that you normally can't with the Reader version.
Access the website to download a trial, then use the Security from the menu ribbons to assign a password to your PDF file.
Official website: https://www.foxitsoftware.com/pdf-editor/#protect-sign