The user config that can be set from the "Settings" menu does not affect the network config file.

When installing a machine with custom cloud-init configuration, the "custom config" combo box only changes the User Config, the Network config box remains as a list of commented lines. This network config causes problems (on Ubuntu 18.04, cloud-init fails to parse it as a YAML file, causing the whole process to fail). Is there a way to pre-set the value of this box/file?
Also, is there a way to enforce the usage of cloud-init file with a certain template?