This is a bit vague.
Do you mean that the PA is invoicing your company for her services (she's external to the company & providing a service to you)?
It boils down to whether that service would be zero or standard rated if you were buying from a UK based company.
- if standard rate, use 20.0% RC SG
- if zero rate, use NoVAT or Exempt if you prefer it to show on the VAT100
(the nett effect is the same - there is no extra UK VAT to pay)
There are weird rules around Place of Supply - particularly for digital/online services.
If your bored one evening - Notice 714A is a hoot : https://www.gov.uk/guidance/vat-place-of-supply-of-services-notice-741a
Hope this helps.