It deliberately mirrors the parameters and output of lightning-fundpsbt(7) except instead of an optional minconf parameter to select unreserved outputs from the wallet, it takes a compulsory list of outputs to use.
utxos must be an array of "txid:vout", each of which must be reserved or available: the total amount must be sufficient to pay for the resulting transaction plus startweight at the given feerate, with at least satoshi left over (unless satoshi is all, which is equivalent to setting it to zero).
reserve is either boolean or a number: if true or a non-zero number then reserveinputs is called (successfully, with exclusive true) on the returned PSBT for this number of blocks (or 72 blocks if reserve is simply true).
Unless reservedok is set to true (default is false) it will also fail if any of the utxos are already reserved.
locktime is an optional locktime: if not set, it is set to a recent block height.
min_witness_weight is an optional minimum weight to use for a UTXO's witness. If the actual witness weight is greater than the provided minimum, the actual witness weight will be used.
excess_as_change is an optional boolean to flag to add a change output for the excess sats.
On success, returns the psbt it created, containing the inputs, feerate_per_kw showing the exact numeric feerate it used, estimated_final_weight for the estimated weight of the transaction once fully signed, and excess_msat containing the amount above satoshi which is available. This could be zero, or dust. If satoshi was "all", then excess_msat is the entire amount once fees are subtracted for the weights of the inputs and startweight.
If reserve was true or a non-zero number, then a reservations array is returned, exactly like reserveinputs.
If excess_as_change is true and the excess is enough to cover an additional output above the dust_limit, then an output is added to the PSBT for the excess amount. The excess_msat will be zero. A change_outnum will be returned with the index of the change output.
On error the returned object will contain code and message properties, with code being one of the following: