There is no script nor any REST api that can be used to create add-devices rule.

On Android-Enterprise platform, there are jpreparation work to complete properly before a device can be enrolled successfully.   It can be a nightmare to have one add-devices rule for one device.  There might be some workaround in certain limitation(s) are imposed, e.g. whether your devices to be enrolled be using no, different or  the same Managed-Google/Managed-Google-Play account, or residing in the same/different device group, etc.  For example, if each device to be enrolled is going to be in a unique device group, using one add-devices rule followed by device move REST api call may be a possible workaound .

Thank you for the Reply Raymond, This is clear to me. However we have a centralized Deployment server and locations to request to add devices to the Central MDM and I wanted to automate the creation of an add device rule once the request was approved. 

It seems this is not possible. 

Thank you