to create a new Agenda item via commandline, you would need to know how many entries that already existed.
e.g.
List Agenda Thru
-> commandline feedback shows 10 entries
Store Agenda 11
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However here is a plausible workaround for your scenario:
Prerequisite:
1. Manually create a new Agenda item with all necessary settings to switch your "Backline work" on, using an expired date/time
2. Manually create a new Agenda item with all necessary settings to switch your "Backline work" off, using an expired date/time
3. type "List Agenda thru" and notice the index of your last two entries you just made.
e.g assuming you originally had 10 entries in the agenda, and the two new on/off agendas was created as index 11 and index 12, create a macro to be operated, that goes something like this:
#create a copy of your known agenda items
CD Agenda
Copy 11 + 12
Paste
CD Root
#get input from the user in proper format
Setvar $AgendaDate= ("Date? e.g 12/31/2020")
Setvar $AgendaTimeOn= ("Start? e.g. 20h30m)
Setvar $AgendaTimeOff= ("End? e.g. 22h0m")
#adjust the settings of the known agenda items
Assign Agenda 11 /First=$"AgendaDate" /Time=$"AgendaTimeOn"
Assign Agenda 12 /First=$"AgendaDate" /Time=$"AgendaTimeOff"