They don’t have a feature in that app to swap from onchain to lightning. Your options are:

1. Swap using a service such as nostr:npub1psm37hke2pmxzdzraqe3cjmqs28dv77da74pdx8mtn5a0vegtlas9q8970

2. Swap yourself using something like Muun wallet

3. Swap using a custodial service such as Strike or River