This is done in the inputs.conf file.
On the forwarder, edit
$SPLUNK_HOME/etc/system/local/inputs.conf (create the file if it does not exist, but ensure the owner and group is the same user Splunk runs as on that system). Add a section like this:
[monitor:///path/to/your/app/log/directory] disabled = false sourcetype = yoursourcetypehere index = yourappindexhere
Change the path to the appropriate directory for your application logs. Also, name the sourcetype with the name as you want it to appear in Splunk as the sourcetype, such as the name of your application - which would allow you to provide these logs from multiple systems for the same type of application with them all as the same sourcetype. If you want it to go into its own index, specify the name of the index, which must be created on the indexer, on that line. If you want the data to go into the default index, remove or comment out that line.
If you want to specify a specific name format for the log file in a directory full of other files, then add this line to the above stanza, as well:
whitelist = *.log
(Or whatever regex you need to match your filename pattern like
".log" is not a valid regular expression because "" is a quantifier and must be preceded by a pattern.
"myapp*.log" is a valid regular expression, but it probably doesn't match the way you want. It literally reads as
(anything, "myap", zero or more "p" characters, ANY character, "log", anything)
The regular expression you probably want is
which reads as
(anything, literal ".", "log", end of line)
which reads as
(beginning of line, "myapp", literal ".", "log", end of line)