How to add data pass to PAYG plan (no data) online?

    I have been searching for quite some time how to add 7-day data pass to my PAYG plan online. My plan for this month has no data ($3/month) and I am changing to a plan with data next month, but I need a data pass for the meantime. I used T-mobile chat who was not able to help me to do it online but I believe it should be possible. When I go to "Change Plan", there is no option about data in "Other services available"...

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