Liquidity providers do not provide quotes on all currency pairs and if you are seeking to convert funds between currencies that are not directly quoted, you would need to find a common currency that is quoted against both and perform the conversion using two separate orders. Using CHF and HKD as an example, if you are seeking to convert CHF to HKD you would need to find a common currency such as the USD that can be exchanged with either. Here, you could first convert the CHF balance to USD and then convert the USD proceeds to CHF.
Please follow the below steps to convert CHF balance to USD in TWS: In an empty ticker row, type in USD.CHF (as you would for a stock symbol). Create a BUY order (to buy USD and sell equivalent amount of CHF). Modify the quantity you wish to convert (in terms of USD), select your order type and your desired price. Click transmit once everything looks correct.
Please follow the below steps to convert USD balance to HKD in TWS: In an empty ticker row, type in USD.HKD (as you would for a stock symbol). Create a SELL order (to sell USD and buy equivalent amount of HKD). Modify the quantity you wish to convert (in terms of USD), select your order type and your desired price. Click transmit once everything looks correct.