How to change FB profile used when interacting with another page?

I used to be able to choose between posting as myself or as Oxjam Beeston Music Festival (volunteer organised music festival raising money for charity) for which I am an admin when posting to a public page of another organisation ie pages where you don’t have to join the group or whatever. In the posting box there would be a drop down arrow where I could choose between the two profiles. This seems to have disappeared. Any help gratefully received! Thanks

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