you will need to configure an inputs.conf file on the forwarder to monitor the file location of your log and send it to your splunk server.
in your inputs.conf file on the universal forwarder you would have a stanza something like this:
[monitor:///var/log/logfilename]
sourcetype = logfile
disabled = 0
In this stanza, you basically want to specify the file location of the log file you are monitoring and give it a source type. You can name the sourcetype anything you want, just name it something that makes sense for your environment.
You can deploy this inputs.conf file in a couple of different ways. If you are manually configuring everything, you could locate this file in the /etc/system/local area under the universal forwarder file path. If you wan't more granular control you could deploy this configuration as its own app to the universal forwarder in which case it would live under the /etc/apps/app-name/ area under the universal forwarder file path. You can name the app anything you like, it is good to have a functional naming scheme so you know what your apps do just by looking at them. This gets into a whole other area of splunk configurations. A good guide to look through is the splunk "Getting Data in Correctly" guide.
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