I create a transaction with inputs from a given address / public key without signatures. Every input needs to be signed. I use a different method to generate the signatures so I can't provide a private key to sign the transaction. Much like the blockcypher API I want to extract a list of strings (called "tosigns" in blockcypher API), sign them externally and combine them with the partial transaction I generated before.
Question 1: What field(s) do I have to take out of the bitcoin transaction json and how do they need to be processed to receive a string (or list of strings for multiple inputs) that just needs to be signed?
Question 2: How do I intergrate the generated signatures with the transaction object to receive a valid, signed transaction, that can be send to the network.
Any JS libraries that help with it are appreciated.