This link https://learn.microsoft.com/en-us/sql/sql-server/install/configure-a-report-server-for-e-mail-delivery-ssrs-configuration-manager?view=sql-server-2014
indicates that you can't use the GUI to set those additional parameters, but that you need to edit the file directly.
(Optionally) Use a text editor to specify additional settings in the
RSreportserver.config file. This file contains all of the
configuration settings for Report Server e-mail delivery. Specifying
additional settings in these files is required if you are using a
local SMTP server or if you are restricting e-mail delivery to
specific hosts. For more information about finding and modifying
configuration files, see Modify a Reporting Services Configuration
File (RSreportserver.config) in SQL Server Books Online.
After a bit more digging around, I found a few articles that mention that it's just not possible. One work-around is mentioned on this other Stack Exchange answer, where you are basically setting up IIS to be your own private relay service.
The quote below is from that answer, by BateTech.
From what I have read, it sounds like you might have to setup a local
SMTP relay server that you send the email through, b/c SSRS requires
that "The Report Server service must have Send As permissions on the
SMTP server" and does not appear to give any options for entering a
password. I am assuming the SSRS subscription email setup only works
using Windows authentication since it uses the SSRS Service, but the
configuration documentation is vague.
Here is a link that shows how to setup the SMTP relay server
Here are the steps you'll need to do differently.
Item 19, choose Basic Authentication, enter your gmail account info,
and make sure TLS encryption is checked. Item 21, enter your server
name as the Fully-qualiified domain name. Enter "smtp.google.com" as
the "Smart host". Then specify the SMTP server you just created above
in your SSRS SMTP configuration.
You probably also need to enable POP3 on your gmail account if you
have not already.