Add 3d line to my address

Hello, I've been trying to add a 3d line to my address (the flat number), but there is no way to do it. I want to register for self-employment and I want to add the flat number as the UTR is sent by post. I contacted 0300 200 3600 but no help, they said add it manually. There is no way to dd it manually. I enter the postcode then I have to choose between different house numbers on my street , but no way to add the flat number. The registration form has 3d and 4th lines but they remain empty as there is no way to add it manually. Any ideas? Thanks.

I found the solution myself and I add it to help others. The solution is to click Unknown postcode and then you can add it manually.

Hi gorillaki,
If unable to amend your address online you will need to contact HMRC to update:
Self Assessment: general enquiries
Thank you.