The call statement calls an Input Method Extension (IMX) DLL. Functionally it behaves like the use statement, and can be used anywhere the use statement is valid except after the begin statement.
The store given must be referencing a function in an external DLL, as below:
store(DLLFunction) "myimx.dll:KeyEvent" + 'a' > call(DLLFunction)
The call statement was introduced in Keyman 6.0.