You can use this event for when you select something from the combo box:
https://wiki.multitheftauto.com/wiki/On ... oxAccepted
Also, guiComboBoxGetSelected requires a combo box element to get the selected item, like this:
function hpFunc ( )
local item = guiComboBoxGetSelected ( _cmbbx )
local text = guiComboBoxGetItemText ( _cmbbx, item )
if ( text == "Regeneration" ) then
guiSetVisible ( regHP, true )
end
end