<rule name="OnlyOffice_Proxy_Inbound" stopProcessing="true">
    <match url="^onlyofficeapi/(.*)" ignoreCase="true" />
    <serverVariables>
      <!-- Nginx $the_prefix için -->
      <set name="HTTP_X_FORWARDED_PREFIX" value="/onlyofficeapi" />
    </serverVariables>
    <action type="Rewrite" url="http://127.0.0.1:8080/{R:1}" appendQueryString="true" />
  </rule>

In this url rewrite rule i send /onlyofficeapi to prefix. ds-docservice.conf uses this prefix value and return true url.