When we have a nested object that needs to be merged with the parent object we can use the formula given below.
In this example, "custom_properties" is the parent object and "properties" is the nested object where we need to merge the "properties" object with its parent object that is "custom_properties".
Soultion: We can use the formula {substr: {json: {$.nested_object_name}}, 1, -1}
For the above example we use {substr: {json: {$.properties}}, 1, -1} which will merge the "properties"(nested object) with it's "custom_properties"(parent object).