I am trying to create a Sales Tax Liability Report because the current system report does not include all of the data that i need to file my monthly sales tax report. 

I created a custom field (checkbox) in the Zoho Books Invoice Module called "Tax-Free". 

I have created a custom report in Analytics including the following columns:

Invoice Number

Company Name

Total Sub Total (BCY)

Total Total (BCY)

Sales Tax - Calculated from 2 fields above

Total Adjustment (BCY)

Filter on Invoice Date (Month)

I would like to add the custom field from the Zoho Books Invoice Module "Tax Free", so that i can subtotal the invoices on this basis for my tax reporting. 

How do I add the custom field in Analytics?