Currently its generated at send-time https://github.com/lightningdevkit/ldk-node/blob/main/src/payment/spontaneous.rs#L82 but in some cases the app needs to be able to specify the preimage.
For example, NWC allows specifying a preimage as one of the request fields when doing a keysend payment: https://github.com/nostr-protocol/nips/blob/master/47.md#pay_keysend
Currently its generated at send-time https://github.com/lightningdevkit/ldk-node/blob/main/src/payment/spontaneous.rs#L82 but in some cases the app needs to be able to specify the preimage.
For example, NWC allows specifying a preimage as one of the request fields when doing a keysend payment: https://github.com/nostr-protocol/nips/blob/master/47.md#pay_keysend