You can't as the function is local!
local function OnTravelToLeaderPromptReceived()
And the local function OnGroupMemberJoined() as well in line 790
And overwriting wouldn't be good at all. You could use ZO_PreHook or ZO_PostHook/SecurePostHook (chekc the wiki for them), but this is also not possible here.
So you cannot overwrite it properly and if you would do it you'd most probabyl kill other addons and ZOs vanilla UI
The real call to the popup is done in line 758 btw.
https://github.com/esoui/esoui/blob/...layer.lua#L758
But it queues the prompt so if you hook or try to hook here you most probably will kill the queue of all popups.
So unfortunately: No chance afai can see