I don't know anything about Harmony remotes, but I am at a loss to understand what "set for the play button" means. Have you replaced the assignment of the normal Play signal with Slow Play? Assigned both codes to the Play button? Does the Harmony recognize long presses and send different signals for short and long presses?

In any case, this is the Discrete Code Hunter section of the forum, and the questino of which code to use has ben answered. Now it would probably be better to post in the Harmony section, where there may be folks who know about Harmony remotes.