How to mark a certificate as a trusted introducer

To mark an certificate as an exportable trust introducer (respectively a certificate authority) for the domain example.com with a preconfigured trust depth of one, use sq pki vouch authorize as follows:

$ sq pki vouch authorize --certifier $FPR_CERTIFIER --cert $FPR_TO_BE_CERTIFIED --domain=example.com

More information can be found here: Chapter 'Certify and authorize certificates'