Inside of the Medical service section of Admin panel, search for device. To create a device, take a look at the POST method of /device endpoint.

All attributes for the object that needs to be provided are optional, but the one attribute which is used inside of the Patient portal is "display".

After calling POST method, an object with attribute "id" is returned.

Use this id everywhere where you need to reference this device.

Usually when device is needed to be submitted, it is found under the "device" attribute; provide this "id" as it's "reference" sub-attribute. See example at: Activity observations