Access to T16 in Shanwick

[falconair]

I am trying to find out the following :

In our route manual, under RAR North Atlantic, it says
that "access to tango routes in Shanwick OCA requires an approval
from the London FMU" via AFTN/SITA telex.

Is this procedure valid, and if so, how does it work if you want to
operate from the Canaries to northern Europe?
What is the correct procedure to follow and how long before do we
have to apply for approval?

Thanks for helping in this matter.