I think it would be nice to add new feature to frontend scripts, which will be fimiliar to macros {ALERT.MESSAGE} and {ALERT.SUBJECT}. Because sometimes some frontend scripts requires a few conditions, which cannot be provided by built-in macros. So to do it's required to create few the same frontend scripts, what provides a mess.

I would provide variables (which can be provided in script by new built-in macro e.g {SCRIPT.INPUT}) in "Confirmation message".

Example situtation:

Creating frontend script with type webhook to add new period maintenance in Zabbix. On host menu would be visible action "Create maintenance to this host for X days". After clicking that would be visible popup with text area with custom message eg. : "How long your host should be in maintenance". When intiger value would be provided then this value will be contained in special new built-in macro.