Setting a default value for one custom field without autocomplete is very simple. All you have to do is to select the necessary option(s). For instance, like in the picture below: 

For the select custom fields with autocomplete you need to define a label and an option as well, see KPOption details for reference.

You can use following configuration as an example: 

In the example above the custom field is rendered as an issue picker and the value is the id of the issue that you want to select.

After the value is set to default, the configuration is updated according to the renderer. You can see example below:

{"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"}}

See also

KPOption