Notifying HMRC re offshore trusts

There are two reporting regimes requiring HMRC to be notified when a UK resident domiciliary settles an offshore trust - s.218 IHTA and Sch 5A TCGA.

Does anybody know if these notifications are just by letter? I can’t find any form. The value is nominal and the settlors excluded so no need to file IHT100.

tsem10105 mentions the Sch 5A obligation without saying how a return should be made.
TSEM10110 is ominously entitled “Non-resident trusts: new non-resident trust: use Trusts Registration Service” but is empty.