The UCAS form does not require you to mention LNAT.
However, the extra Oxford application form requires an LNAT Registration Number to be entered.
So you should register for the LNAT and book a test slot before filling in your Oxford form, and enter the LNAT Registration Number on your Oxford form.
You can send in your UCAS form either before or after registering for the LNAT. If you do it after, you should add your UCAS application number to your LNAT file later (by logging back in to the registration system).
Follow these instructions and you are all set.