InstallOptions after folder selection

I've a small problem with an Install Options page which will be displayed after the folder selection dialog.
The new dialog will only be shown if a specific section is selected.
Everything works fine, except:
I can't change the text of the "Install" button on the target location page, it's always "Install". But I want it to be "Next >" if a specific section is selected.
I've already tried the following, but it did not work:
GetDlgItem $R0 $HWNDPARENT 1
SendMessage $R0 ${WM_SETTEXT} 0 "STR:Next >"

SendMessage work properly, I've tested it by sending WM_CLOSE to the button, then id dispppeared.
It looks like the Next/Back buttons are set by NSIS after calling .OnNextPage/.OnPrevPage, I could not find any way to change these buttons in .OnNextPage / .OnPrevPage or do I've missed something?

Put that SendMessage stuff in .onInitDialog.

Thanks, that did work. 🙂
Maybe adding "section-toggled" installoptions page after the target location page to ${NSISDIR}\Examples\Modern UI\InstallOptions.nsi would be a good idea.