Posting on another Facebook Page as your own Page account name?

On Facebook if you own your own page

And then you want to post something on someone else's page .... how can you do that? I did it before but I forgot

For example I designed a poster of say Spiderman, and I post it on my Facebook Page and then I want to post it on the Spiderman Page - I think it's something like you have to write on your own Page wall ... @Spiderman then blah blah

But I can't remember

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If I post it as my user account then it seems to work but then I go back to check, I don't see it

And anyways I want to post it as my page ...

Like I said done this before but forgot how to

Anyone know? Thanks