Write an ABAP formula for a mass change

Hi 
I americium struggling with the wide alteration pop-ups that allows to make formulas and ABAP formulas.
I americium utilizing transaction IP17 (change attraction items), paper enactment "Goto" / "Carry retired a wide change". I prime a tract which contains escaped substance (maintenance point text), and I privation to make a look to manipulate the contented of this substance tract (i.e. remplace a portion of the text, for illustration update "3MO Int Maint" into "1MO Int Maint") for galore items.
I recognize this is not the intent of the "replace only" emblem which is simply a filtering relation for the enactment and not a substance contented manipulation function, but it seems imaginable to usage a formula, and constitute your ain ABAP formula, which opens a pop-up to constitute ABAP codification with the pursuing template:

* X is the worth you indispensable return
form GET_NEW_MPLAN_MPOS_MASS_PSTXT
using conception operation MPLAN_MPOS_MASS
changing X.

endform.

I person tried galore things but erstwhile I enactment a worth successful the X adaptable (as basal arsenic X = 'TEST'), the look cheque says it has nary error, but past the wide alteration fails. Error connection is: "Error@ Maintenance Item 63970 : Obligatory parameter PSTXT of attraction point is not filled ( API Message MPLAN 214 )". Which I bash not recognize arsenic PSTXT is the tract I person selected for wide alteration (maintenance point text).

Screenshots of each measurement attached.

What I americium doing incorrect here?
Thanks successful beforehand for nay idea.