If a PD who does not support LLDP while needs 15w power connected to an EX3400-48P switch, it will be classified to class 0 by default.

We would like to know how to allocate 15 or 30 watts manually to a port.

There are only two mode of POE supported.

1.class mode(default)

In class mode, Power is allocated dynamically using the classification process. (This is the default mode).

The PoE controller does not allocate power to a port until a powered device is connected. The class of the connected device determines the amount of power allocated. Thus in class mode, any PoE port can be used to power a device and all the PoE ports on the switch can be used as long as the combined power demand does not exceed the PoE power budget.

2.static mode

In static mode, Power is allocated based on the maximum power to the PoE ports in the order of port number.

By default, the priority of POE interfaces are low. If you set priority to high for one POE interface, it will be allocated power with higher priority. The interfaces without higher priority setting and the power priority are determined by the port number, with lower-numbered ports having higher priority. If the PoE power budget for the switch is insufficient to provide maximum power to all the PoE ports, the remaining interfaces cannot be allocated with the power.

To set the management mode to static and allocate specific power to a port.

#set poe management static

#set poe interface ge-0/0/0 maximum-power 30

#set poe interface ge-0/0/1 maximum-power 15

NOTE: If the PoE power budget for the switch is insufficient to provide maximum power to all the PoE ports, we recommend that you do not change the management mode from class to static. If you change the power management mode to static and do not change the other default settings, the PoE controller allocates maximum power to the PoE ports in the order of port number, which means PoE will be disabled on higher-numbered ports when the PoE power budget runs out.

https://www.juniper.net/documentation/us/en/software/junos/poe/topics/task/poe-cli.html