requesting specific revision via REST API doing a strange thing with workspaces/streams

I am looking at this page, the section describing how to request a build on a specific VCS changelist revision:
https://www.jetbrains.com/help/teamcity/rest/start-and-cancel-builds.html#Run+a+Build+for+the+Specific+Changes+Revision

It says this is the format:
<build branchName="feature1">
 <buildType id="buildConfID"/>
 <revisions>
     <revision version="57ef8447c7e277eda9d4e3ce433795d5eb755a64" vcsBranchName="refs/heads/feature1">
         <vcs-root-instance vcs-root-id="vcsRootId"/>
     </revision>
 </revisions>
</build>

When I use this format, it does start a build - but it's syncing the workspace to a different perforce stream than what I expect.  It does ‘Update Sources’ and replaces the previous workspace contents with that of a different depot/stream in our perforce. When I look at the app/rest/builds/id:xxxxx page for the build compared to one started from the UI, I cannot see any difference.  The streams and vcs roots are all identical.  

EDIT: Upon further examination, it seems that our VCS Root has a “Stream” and then there is a checkbox “Enable feature branches support” which is checked.  Then it says “Edit branch specification:” and in the text field is another stream listed.

The stream I WANT, and which the UI uses, is the one listed in the Edit branch specification.  But the one being used by the REST API call is the main Stream setting.

Is there any way that  I can tell it to use the stream specified in the Enable feature branches support, when using the REST API?

Thanks for any suggestions!