Thanks michael!
Just tested it outside of the project by writing the modul from the bottom up.
It is indeed necessary to copy the content of the text object to a string.
But only the first one.

str_stri(string1,string2)

string1 needs to be a string, while string2 can be an element of a text object.

str_stri(object_tmp,format.string[i]) is thus a valid operation which I can use now.

Thanks again!