VSTS 2013: Custom Build

Question

In TFS2013, the build process is defined by an XML File. I've taken the TfvcTemplate.12.xaml and created a customized version of it.

In the Custom Version, I have a couple extra steps added for after the DEPLOY step.

Pre-Test
Test
Post-Test
Pre-Build
Build
Post-Build
Copy Binaries
Post-Copy Binaries <== Custom Step

In the Post-Copy step... I have toggles and credentials to control parts of the post-build - copy files? true false, deploy website? true false, account information

In Visual Studio, the screen that pops up uses these extra fields. You can then adjust those fields at request time:

But on the TFS2013 website, it doesn't have a place to edit those "choices":

What's the process to add those Costume Build Parameters to a website request so that an administrator (AKA: Not me, the programmer) can request a build and use those extra TRUE/FALSE fields?

PS: There is movement towards TFS2019, but in my company it'll probably be another year for that to be live and usable.