Luci keep a taboption value and use it as defaut for an option

I'm looking for keep the "prefix" value for use it in the option bellow.
For, at each new line add (button [Add]), the default for "rule" is equal to the "prefix" value
(Maybe I need to read it after have save it?
See lats line bellow.
Thanks to all !

s:tab("mytab", "My Prefix", "")
	prefix = s:taboption("mytab", Value, "prefix", "Prefix name")
		prefix.datatype = "string"
	tp = m:section(TypedSection, "topics", "Rules")
		tp.template = "cbi/tblsection"
		tp.anonymous = true
		tp.addremove = true
	-- MQTT topic rule name
	rule = tp:option(Value, "rule_n", "Name rule")
		rule.datatype = "string"
		rule.rmempty = false
		rule.default = ###????### = prefix from taboption?