One approach would be to tell VA in its options dialog to use a custom list of include path's, and add the OpenSSL directory list to this list of path's, but this would effect all of your solutions.
Are you aware that the IDE supports a "CppProperties.json" file when you are opening a folder? This page explains about it:
I have set up a simple test case here, and ended up with the "CppProperties.json" file:
"name": "Windows x64",
VA is picking up and parsing this file, since when I then have a #include line to a header inside one of these library directories, VA knows where to find the file, and is parsing it.
So long as the location of your OpenSSL library does not change to often, you can set up this file and just forget about it most of the time. Without needing to create a Visual Studio solution, you can use this JSON file to tell the IDE about the specific settings that it does need to know about.