Setting a default value for one custom field without autocomplete (checkbox, radio buttons, single or multi select) is very simple. All you have to do is to check/select the desired option(s).
Example:
Setting the values for the select custom fields with autocomplete is not that easy, since the options are of type KPOption, so they need to have defined a label and an options.
The configuration for such a field would be like follows:
In the example above, custom field is rendered as an issue picker and the value is the id of the issue I want to select.
After the value will be set to default, the configuration will be updated by itself according to the renderer. In our case, it became:
{"option":{"label":"","value":"10000"},"meta":{"longLabel":"TST-1 - update the documentation","tooltip":"update the documentation","shortLabel":"TST-1","iconUrl":"/secure/viewavatar?size=xsmall&avatarId=10310&avatarType=issuetype","href":"http://localhost:8070/browse/TST-1","labelOverride":"TST-1"}}
If you want to set multiple values to a field, the values should be separated with #|# separator. Example: {"label":"","value":"10000"}#|#{"label":"","value":"10001"}