GetFieldBuffer() is used to return the input buffer of the field specified as the method argument. The field specification can optionally include a prefix:
LET cur_buff = DIALOG.GetFieldBuffer("mytable.mycolumn")
There is some specifics about using variables with ui.Dialog methods.
You can learn about them here.
The input buffer is usually specified by fgl_dialog_setbuffer() function and DISPLAY TO / DISPLAY BY NAME statements.