You can find the backend's URL suffix with grep
:
grep "frontName" app/etc/env.php
(
o=(
--default-character-set=utf8
--host=<host>
--password=<password>
--user=<user>
-D <database>
-e "SELECT path, value FROM core_config_data WHERE path LIKE 'admin/url/%'"
)
mysql "${o[@]}"
)
The full Magento 2 backend's base URL is <frontend base URL>
/<backend suffix>
.
If you are uncomfortable with the command line, then you can find the backend's suffix using GUI.