You can only claim for treatment that has already happened. If you have paid for treatment for your pet in advance, you can’t claim for this treatment until it has happened.
For example, if your pet has been recommended 10 sessions of behavioural treatment, but you’ve only had one session so far then you can only claim for that session - even if you’ve already paid for all 10 sessions.
To claim for the individual sessions, your vet/behaviourist/therapist needs to issue a separate invoice for that individual session.
Alternatively, you can wait until all the sessions are complete and submit a claim for all of them in one claim.
Please note: you have 6 months to submit claims from the treatment date so if the sessions are taking place over an extended period of time it might be worth claiming for the sessions sooner to avoid any risk of them not being covered.