This is the simple code which will show you as to how you can call the external routine from the current program.
Program 1
REPORT ZSOURCE2307.
* List of the current program
WRITE / 'I am program ZSOURCE2307'.
* External perform
PERFORM EXTFORM IN PROGRAM ZSOURCE2308.

Program 2
REPORT ZSOURCE2308.
* Form definition
FORM EXTFORM.
WRITE / 'I am extform in program ZSOURCE2308.'.
ENDFORM.

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