Disable Specular Highlights/Reflection in surface shader

In the standard shader, there are these 2 options to check "Specular Highlights" and "Reflections". I am writing my own surface shader with standard lighting and I can't figure out how to implement these 2 options. Specifically, I want to disable environment reflections while still enabling ambient lighting. I've tried:
Setting "Reflection Probes" to "Off" on my MeshRenderer does nothing. Setting SurfaceOutputStandard.Occlusion to 0 in my shader disables both ambient lighting and environment reflections.
Adding
#define _SPECULARHIGHLIGHTS_OFF 1
to my shader does nothing.

Is there a tag or define somewhere? Is this even possible? I'm on Unity 2019.2.11

 

Hello, not sure you ever find a solution, but I had the same problem and this solved it to me, I found it on the standard shader code.

Code (csharp):

1.  
2. Properties {
3. ...
4.   [ToggleOff] _SpecularHighlights("Specular Highlights", Float) = 1.0
5. ...
6. }
7.  
8. Subshader {
9. ...
10.         #pragma target 3.0
11.         #pragma shader_feature_local _SPECULARHIGHLIGHTS_OFF
12. ...
13. }
14.