How can I mail merge a date from an Activities/event directly in an invoice template?

How can I mail merge a date from an Activities/event directly in an invoice template?

Hi everyone,

We are a training company and for each contact we need to create several training dates, as activities in Zoho, that will automatically create events in our Google apps calendar.

Once the training is completed we need the dates when the training has been delivered to show up on our invoices.

Is it possible to customise an invoice template with "date field" from a specific event or a series of events linked to the contact I am creating an invoice for?

I have tried the below fields inside an invoice template, without any success.

${Closed Activities.Start DateTime}

${Activities.Start DateTime}

I have also tried to add a new field inside an activity, called "session date" as a date field (please see it below), but it is still not picked up when try to create an invoice.

${Activities.Session Date}

Can you please let us know if this is possible and how to achieve it?

I look forward to hearing from you

Best regards,

Enrico