Adding VAT to sales journal

Hello,

I would like to use the sales journal in NAV 2013 R2 to record my monthly revenue (selecting customer name, amount and balancing to revenue account).

I cannot figure out how to have the VAT calculated automatically based on the amount I've entered.

I have the purchase invoices setup properly so that when I pick my VAT posting group it correctly calculates and charges the VAT but I can't seem to get this to work in the sales journal.

Any help would be appreciated.

Thanks in advance!