I ran into the exact same problem under identical circumstances. I don't have the tnsnames.ora file, and I wanted to use SQL*Plus with Easy Connection Identifier format in command line. I solved this problem as follows.
The SQL*PlusĀ® User's Guide and Reference gives an example:
sqlplus hr@\"sales-server:1521/sales.us.acme.com\"
Pay attention to two important points:
I found these good questions to detect service name via existing connection: 1, 2. Try this query for example:
SELECT value FROM V$SYSTEM_PARAMETER WHERE UPPER(name) = 'SERVICE_NAMES'