When you create a forecast, you can change the default settings around VAT/GST/Sales tax for this specific forecast.
In the forecast, click Controls in the top left.
In the dropdown, click Settings.
The settings then appear to the right.
From this page, you can update the following:
If the organisation in question is liable for VAT/GST/sales tax
The VAT/GST/sales tax scheme: Cash or Accrual based
The VAT/GST/sales tax control account
The VAT/GST/sales tax frequency
The end of the VAT/GST/sales tax period
The default VAT/GST/sales tax rate
VAT/GST/sales tax payment terms
Any change made will impact the specific forecast in question and any prediction created from it.