I want to implement that on my wallet, but apparently there is no spec or howto guide. Why?
Gah, can't believe I didn't link the primary source. Updated so BIP 78 shows above the fold now.
What are you looking for in a howto?
This merged send payjoin PR for the bitmask web wallet could be the basis of a how to: https://github.com/diba-io/bitmask-core/pull/121
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