Remember how each "form" in Access has it's own specific Record Source?
A similar situation exists with FileMaker. The current record on one layout will not automatically be accessible on another layout unless both refer to the same "box" (called a table occurrence) from Manage | Database | Relationships in Layout Setup | Show Records From.
You have to "put the ID value in a bucket" carry that bucket to the other layout. Create a new record and take the ID out of the "bucket" and put it into the appropriate field of the new record in the related table.
Set Variable [$ID ; value: Cases::CaseID ]
Go To Layout ["Transactions" (Transactions)]
Set Field [Transactions::CaseID ; $ID ]