Currently, only the Contact object records can be linked as Signer for Box Sign docs. Can you please consider having this feature for custom objects.

Here's the document for reference:

https://support.box.com/hc/en-us/articles/19555153787411-Setting-Up-Advanced-Sign-Settings#:~:text=To%20send%20the%20signature%20request%20to%20a%20pre%2Dpopulated%20recipient%2C%20the%20field%20must%20be%20populated%20with%20a%20Contact%2C%20and%20the%20Contact%20must%20be%20populated%20with%20a%20valid%20email.