In that case, your two last digits of the PRN are 00.
For the first digit, it depends what dimension your mailbox is in. Scroll down to see the dimension numbering.
Heres how location numbers work.
The most important locations have something called a PRN (Postal Routing Number), It is made up by these rules:
The first number is the number of the dimension, the last two digits are the place number. Both are assigned by Balgov.
1 - Overworld
0 - Nether
9 - The End
Contact Chaz, the minister of postal services. If the place is important to the server, it will be assigned a unique PRN.