json format for the snmp-dca-base user-defined part

The question: When I'm constructing the json for the payload to create an snmp-dca-base monitor, how do I phrase the user-defined parts?

I am on 11.5 trying to add a custom snmp-dca-base monitor with user-defined data so that I can poll my custom oid. (Not using snmp-dca since we want to poll something more specific.) I've beaten my head against this for a while and do not understand the bit in the manual which says how to format this.

Adding stanzas like "user-defined key1 value1 user-defined key2 value2" works in tmsh on the command line but I've hit a wall with the json.

Page 49-127 of the 11.5.0 tmsh reference says (it seems copied from the external monitor section?):

user-defined
Specifies any user-defined command-line arguments and variables that
the external program requires. Use the following syntax to specify a user
defined parameter.
modify external my_external user-defined my_param_name
my_param_value
Use the following syntax to remove a user defined parameter.
modify external my_external user-defined my_param_name none

For a monitor that would look like this without the user-defined portions when created from scratch (this one works):

ltm monitor snmp-dca-base /Common/cwt2 {
    community bigsecret
    defaults-from /Common/snmp_dca_base
    interval 10
    time-until-up 0
    timeout 30
    version v2c
}

I am trying to make it look like this if I created it from scratch (this one does not work):

ltm monitor snmp-dca-base /Common/cwt2 {
    community bigsecret
    defaults-from /Common/snmp_dca_base
    interval 10
    time-until-up 0
    timeout 30
    version v2c
    user-defined thing1 .1.2.3.4.5.6
    user-defined thing1_coefficient 123
    user-defined thing1_threshold 456
}

My json input payload so far (I get the least complicated error message with this one):

{"community":"bigsecret","version":"v2c","user-defined":[{"cwt2_coefficient":"1"}],"defaults-from":"/Common/snmp_dca_base","name":"/Common/cwt2"}

But the output:

{ "errorStack" : [], "message" : "\"{\" unknown property", "code" : 400 }

And the log line from /var/log/audit:

Mar 2 15:01:52 lb-lab notice icrd_child[25573]: 01420002:5: AUDIT - pid=25573 user=cwood folder=/Common module=(tmos) status=[Syntax Error: "{" unknown property] cmd_data=create ltm monitor snmp-dca-base /Common/cwt2 { community bigsecret version v2c user-defined replace-all-with { {

It thinks that the '{' (left squiggly bracket) is a property? Odd, but I do get the monitor created when I remove the user-defined portion from the input json, albeit without those lines.