IATA airport slots for GA aircraft can be requested using a standardized machine readable request message. These messages are called General Aviation Clearance Requests (GCRs). Depending on the authority managing the airport slots, there can be an email address or a web interface.
When autorouter considers a flight to require one or more IATA airport slots, this dialog becomes available which gives you the matching GCRs. You can copy the messages to your clipboard or in case autorouter knows about a suitable email address, open the message in your default email program registered with your browser.
Once you submit the message, there should be a response. If the message can be processed automatically, it is typically fairly quick (e.g. a minute) but if the message cannot be automatically processed as there is no suitable slot available or it is somehow not acceptable, processing time can vary depending on the human operator.
A successful response contains an airport slot ID which you have to copy to the “Slots” field in the “Additional inputs” section of the route. The slot ID starts with the airport ID followed by additional characters (usually 10 digits).
Email generation is currently only supported for coordinated airports in Germany.