Probably. How it is not obviously painted - like that you should give it to them. Maybe you provide them with the source code for signing the document, and then use a purely identifier so that they understand what certificates to sign?
well, look at the top of the documentation
https://depositsign.com/swagger-api-doc/index.html - there we are talking about some kind of encryption
that is, we need to clarify - you provide them with clientid / sources, they give us username|password, and we will need when requesting:
1. encrypt the password with their algorithm and certificate
2. with this login/password, knock on authorization /api/v1/{clientId}/auth/login where and get a token for subsequent requests?