This is called a direct reference. Since version 19.3, pip support this in both command line and requirement files. Check out an example from the official documentation.
As to OP’s question, simply put the local wheel’s relative path, i.e., ./<my_wheel_dir>/<my_wheel.whl>, in requirement.txt, e.g.,
./local_wheels/ABC-0.0.2-py3-none-any.whl
Flask==1.1.2
flask-restplus==0.13.0
gunicorn==20.0.4