When adding BFPO addresses, HMRC require you to use the full 'traditional' postcode format to ensure gift aid can be claimed on donations received from the constituent.
Enter the BFPO address in the address field, putting the BFPO number in the Town field.
The postcode should be looked up from the full list of conversions between the numbers and the full postcode available on the MoD website.
Click on the link below to open the full conversion table on the MoD website: https://www.gov.uk/government/publications/british-forces-post-office-locations