In all infotypes' records, you can only set the Text exists field ITXEX to 'X'. The program module which processes the infotype, calls a routine to maintain the text.
However in IT 19, it uses the structure RP50M field TEXT1. The MP001900 program uses a module update_text to update text.
Try
a)setting P0019-ITXEX = 'X' and RP50M-TEXT1 to a text string.
b)calling a routine to do what this module update_text does.