All HTTP requests to Microsoft’s Dataverse Web API should include, at minimum, the following four headers:

Accept: application/json 
OData-MaxVersion: 4.0 
OData-Version: 4.0
If-None-Match: null

The last one’s value can be confusing. By default, it should be set to a literal string with the text value of “null”, not a null value (e.g. headers.Add("If-None-Match", "null"), not headers.Add("If-None-Match", null)). (There are some special cases where you’ll deviate from this default value, but in all cases the header itself should always be sent.)