IIS 8.5, rewrite module and backslash in URL

Hi

I have a website that can be reached over a MS IIS proxy. The webserver itself is an apache.
Some pages are referenced with the path containing backslashes. How can I rewrite the URL to pass "\" and not "/" as it happens now ?

https://website.ch/_utils/document.html?user_test/pm-ref%2F10025%2Fep%2F11017%2Flauf%2F1

should reach the apache as

https://website.ch/_utils/document.html?user_test/pm-ref\10025\ep\11017\lauf\1