I found this older thread about the process of reserving a timed entry to Van Gogh museum using the iAmsterdam card. It appears the glitch described in the post has been resolved with a specific link on the iAmsterdam page to making reservations.
Has anyone had more recent experience with this process (successful or not)? I will be in Amsterdam June 20-24 and won't have physical possession of the card until arrival, which means I won't be making timed entry choices until a day or two ahead of the museum visit. Any advice much appreciated.