Since Tera Term 4.59, you can associate with TELNET protocol with installer.
Specify Application like follows.
"C:\Program Files\teraterm\ttermpro.exe" /nossh /T=1 %1
If TTSSH is installed and SSH is enabled in TERATERM.ini, Tera Term starts SSH connection to the remote host.
Use TTSSH command line option "/nossh". It specify the connection protocol does not SSH.
Use ttermpro.exe command line option "/T=1". It explicitly specify the connection protocol is TELNET.