The RICs for spot rates are constructed using the following pattern: currency ISO 4217 code + "=", e.g. "EUR=", "GBP=" etc.
For cross currencies the RICs for contributed indicative exchange rates are constructed using: base currency ISO 4217 code + quotes currency ISO 4217 code + "=", e.g. "EURGBP=", "GBPJPY=" etc. Note that these RICs only exist for currency pairs actively traded in the FX market and the order of the currencies is significant. Typically there's a convention in the FX market which selects one currency from a currency pair as base and the other as quoted currency, hence the RIC "GBPCHF=" exists and the RIC "CHFGBP=" does not exist.
There are also RICs for cross currency exchange rates calculated by Refinitiv from respective spot rates. These RICs exist for most currency pairs and are constructed as base currency ISO 4217 code + quotes currency ISO 4217 code + "=R", e.g. "CHFMAD=R", "MADCHF=R" etc.
You can also use RIC Search tool available from this portal.