Permission constraints by custom field

[craigmoscardini]

I am trying to apply restrictions based on a custom field in the same way as built in fields but cannot find anything that works.

For example a custom field call network_segment and restrict to value ABC

I have tried a few different formats and found the below works but doesn't match any data so shows no sites at all

[{"custom_field_data__network_segment": "ABC"}]

Is there some other format that should be used or is it just not possible to reference custom fields for permission constraints?