Luis,
If you're using the "Customer Invoice for Professional Services" Form Template, it does appear that the Project Task ID you're looking for is within the data schema.
The binding appears to be in the following path:
FormCustomerProjectInvoiceServiceView.CustomerProjectInvoiceProductViewSubtotal[*].CustomerProjectInvoiceProductViewSubSubtotal[*].CustomerProjectInvoiceProductViewSubSubtotalItem[*].ProjectTaskID
It doesn't appear the standard Form Template has a field for this included, but within the schema it indicates the data should be there. I'm also not sure the table actually has the structure to support this, but you could add additional repeating elements to get to this level of detail.