You only need to tell HMRC about conditional growth shares if they have been awarded to employees.
An initial filing needs to be made by July 6th following the tax year in which the options were granted. Once this filing has been made, the scheme should be closed such that subsequent years' filings are not required.
The file required, along with detailed HMRC guidance notes can be found here:
For the notification:
Fill in the third sheet, Other_Acquisition_V3. Most of the questions are reasonably self-explanatory, but a few may need a little help:
2. As this is a bespoke scheme that you have put together not in the aim of tax avoidance, answer "no"
19. Enter "1" for shares
25. Enter "1" as they have restrictions
26. Enter "1" as they bear the risk of forfeiture, if the conditions are not met
27. Enter total vesting period, or best estimate if not specific
28. Enter nominal value of shares, as they have been set at a hurdle to make them worthless on award
29. Enter nominal value of shares, as they have been set at a hurdle to make them worthless on award
30. Enter "yes", if an ITEPA 431 was entered into on share award
31. If yes, then "all"
32. Leave blank
33. Enter aggregate nominal value of the shares
35. Enter "no"
37. Enter "yes"
38-40. Enter "no"
Don't forget to close this scheme once you have submitted your notification.