Download gene expression from mouse atlas for specific brain region

Dear Allen Brain Atlas team,

Thanks for providing all those great datasets and tools! I would like download the expression energy values for a specific gene in a specific brain region. For this I tried using the Gene Search tool, and searched for example for “HTr1a” which gives me the expression energy for 12 regions. Now I would like have this information in more granular detail, so for example in brain regions within the isocortex (for example Orbital area or Retrosplenial area). Is there a way to obtain the expression energy value for specific brain regions via the web application? I assume that this is possible by using the API (and the SectionDataSet via the RMA), but I am not sure how to formulate the query.
Could you give a hint on how to approach this?

Best wishes,
Rosa