You'll need to enter the full (100%) VAT rate upon creating your transactions to receive a 100% VAT payment.
To do that, click on the + New icon at the left pane and select the transaction type that you need. Once done, enter the information needed on the entry and select the full rate of the VAT that you're going to use. Please see the sample screenshot attached for reference:
On the other hand, if you want to add a 100% VAT invoice to your VAT return/expenses, you'll need to enter a positive and negative amount on the line item of your expenses to zero out the VAT. The first amount will contain a vat code, while the negative figure must be set up as NO VAT.
After that, you may manually key in the VAT amount in the sales tax field. This will create a 100% VAT transaction.
I'm also adding these help articles below to give you more insights about adding VAT rates in the system and all the things that you need to know once you're VAT registered: