Status
The registration form does not yet elect custodial. Use Help if this is the required type.
Parties
Minor and adult custodian
Control
Custodian trades until majority
Form
Not on the registration screen yet
Ask
Help desk
Control
The custodian trades until majority under the applicable state statute.
Minor, custodian, and the form
A custodial account is an adult custodian trading for a minor under the state statute that applies, in the UGMA/UTMA pattern. The custodian controls the ticket until majority. The minor is the beneficial owner. That split is the point of the account.
The registration form does not yet offer custodial as a choice. If this is the account you need, use the help desk and do not force it through an individual brokerage application. Full fields for both parties come in a later build.