rsyslog rewrite hostname before relay

I am setting up rsyslog in a multitenant environment to relay to a central server. Because it is multitenanted, I would like to prefix the hostname from the first rsyslog server with a customer specific prepend before relaying on to the central server. I had planned to set the prefix manually, however, the prefix is configured in another file on the server, and if this could be gathered from that file, that would be even better.

Because the first server will be relaying from multiple hosts, the prepend has to be a dynamic rewrite that includes the original hostname rather than a hard-coded overwrite of the same hostname for all entries, which I've seen in some examples.

Ideally, what I am trying do do is summarised by the following pseudocode:

ruleset(name="myrule"){
    set $hostname = "<prefix>-%HOSTNAME%"
    action(type="omfwd" target="remote-ip")
}

I will be responsible for both the intermediate relay and the central server, but each relay can host multiple customers, so I don't think that the rewrite can be done on the central server, but I have full control of both layers. Each customer is connected via a dedicated interface and I was planning for a separate ruleset attached to an input configured for each interface and the ruleset to include the customer specific prefix. For this reason, I think the config needs to be on the relay, but if there's a different way, then I am willing to try anything that meets the end-goal of making events customer-identifiable.

The reason for wanting to use the hostname rewrite is because this is in-line with how other tools are configured in the environment and it is highly desirable to keep a homogenous setup. However, if that is not possible, another method may be considered if the first is not technically feasible.

What is the correct way to do this?

The answer to this question ended up being surprisingly simple. I injected a constant value in to the template like so:

template(name="ForwardFormat" type="list") {
    constant(value="<")
    property(name="pri")
    constant(value=">")
    property(name="timestamp" dateFormat="rfc3339")
    constant(value=" ")

    constant(value="myprefix-")

    property(name="hostname")
    constant(value=" ")
    property(name="syslogtag" position.from="1" position.to="32")
    property(name="msg" spifno1stsp="on" )
    property(name="msg")
    }