How do I set a stop loss order for a position I have?

No, it is currently not currently possible to attach a Trailing Stop order as a Stop Loss in IBKR Mobile.

Possible Stop Loss attachment order types include Stop or Stop Limit…

In IBKR Mobile you can create a new order with an attached Stop Loss order or add a Stop Loss leg to an existing open order using the Attach Order section of the Order Ticket.

To attach a Stop Loss to a new order:…

Yes, to set a Trailing Stop order on IBKR Mobile choose Trail as the Order Type.

Additional Information…

You can attach one-time adjustments to stop, stop limit, trailing stop and trailing stop limit orders. When you attach an adjusted order, you set a trigger price that triggers a modification of the original (or parent) order, instead of triggering order transmission…