Can I add Related Identifiers to the DOI metadata via the Fabrica Form?

You can use the form in Fabrica to include metadata for every possible property in the entire
DataCite Metadata Schema. When you log into Fabrica with your repository credentials, click “create (form)” in the DOIs tab. Fill in the mandatory fields at the top and then scroll down to add recommended and optional metadata. To add a Related Identifier, you must include a globally unique identifier (view the full list of supported unique identifiers here). Once the identifier is entered, the Related Identifier Type is auto-populated, for example, a DOI. You then need to select the Relation Type from the drop down menu. This is a controlled vocabulary list. The Relation Type you chose will determine whether the link is considered to be a citation.

More information can be found here.