Using the REST API to populate a pick list on Clarizen

Hi, 

First of all, as I said in another post, I'm new at using the Clarizen REST API.

I have a specific request for a custom action that would allow all users, no matter their licence type, to select the name of a work item in a pick list and then automatically send an email to the Project Manager to be added as a resource. 

What I thought I would do is as follows: 

1) Create a pick list on the Organization level 

2) Populate that pick list with the names of all active and reportable work items in Clarizen 

3) Send an email. 

The issue though is in step 2) : it doesn't seem to be doable using a workflow/scheduled workflow rule and so I thought it could be done using the API. However, I'm having difficulty grasping the mechanism behind the REST API: 

-Does the JSON request have to be written in a file or in the URL ? 

-Would it be possible to call the API from a custom action ? 

I've already looked through the documentation, both here: https://api.clarizen.com/V2.0/services/ and https://success.clarizen.com/hc/en-us/articles/205711828 but I can't find anything helpful. 

Any help would be greatly appreciated !  And of course, if there actually is a way to do this using workflow rules, I'd be more than happy to hear it :) 

Thanks, 

Amir