Sending a Kibana Alert through a proxy help

Hi there
We have an OT environment that has a requirement to create incidents in a cloud-based ServiceNow instance.
To do this, we have to go through a proxy which requires kibana.yml to be modified to allow this.
We can manually reach the ServiceNow endpoint via a webpage url so we know the proxy & firewall rules are working.
However, when I try to do this via Elastic, I get a range of errors depending on what settings I use in kibana.yml.
Has anybody done this before?
What is the syntax setting for user authentication into the proxy?

Standard settings are:
xpack.actions.proxyOnlyHosts: ["xxxdev.service-now.com"]
xpack.actions.proxyRejectUnauthorizedCertificates: false
xpack.actions.rejectUnauthorixed: false

I am currently setting proxyUrl to:
xpack.actions.proxyUrl: "http://ProxyUser:ProxyPassword@10.16n.nn.nn:80"
but have tried multiple different ways of doing this.

How do I authenticate the user so that the proxy will allow the traffic through?
Thanks
NigelW