{% comment %} A ref-URL trigger: the reference itself, plus the handle the deep link needs. `link_handle` exists because ChannelConnection has no username column and apps/channels belongs to a sibling workstream — see apps/flows/triggers/links.py for the three-step resolution this feeds. Leaving it blank is right as soon as an adapter reports the handle itself; filling it in is what makes the link work today. {% endcomment %}
Letters, digits, hyphens and underscores. It appears in the link and inside the QR code.
Needed to build the link on channels that have not reported their username. Messenger does not need one.