How to run merge agent job step on tsql
In SQL Server parlance, an "Agent" is an external executable that runs outside the SQL Server process. What you see in the job step are the command line parameters for that executable, not a T-SQL script. 

The Merge Replication Agent is actually an executable: replmerge.exe. If you want to execute this from outside the SQL Agent job, you would open the command prompt on the server, and run the replmerge.exe command, followed by the contents of the job step:

replmerge -Publisher [Publisher\Instance] -PublisherDB [Database] -Publication [Publication _name] -Subscriber [Subscriber\Instance] -SubscriberDB [SubscriberDB] -Distributor [Distributor \Instance] -DistributorSecurityMode 1


For more info on the replmerge executable, and the available parameters, see the online documentation.