The custom fields are structured in the same way they are for jobs. The number of fields, the type of fields and the field permissions are defined before you start adding data. This work needs to be done in the admin section of the vWork website.
Once this is done, you can populate the values via the API. To do so, you include the field name and value in a POST or a PUT to the asset.
<?xml version="1.0" encoding="utf-8"?>
<asset>
<custom_fields type="array">
<custom_field>
<name>Last Service</name>
<value>July 15th 2020</value>
</custom_field>
</custom_fields>
</asset> Please note, that the values you supply will need to match the type of custom field in use. For example, you could not populate a number custom field with text.
If you do not have access to the website, you can perform a GET on an asset to see the custom field structure that has been defined for your assets.