So it looks like outlook is considering the : as something like the : from the http: and adding :// to it
At the moment I do not have a solution for you, and I cannot run this and see.
Try options like:
Prefixing the url as
http://ms-powerautomate:/console/flow/run?workflowName=workflowName
or
https://ms-powerautomate:/console/flow/run?workflowName=workflowName
A way to find out is to putting your desktop flow URL manually on the browser address bar and hitting enter. See what the Url changes to and then use that one in the hyperlink.
or this security warning can be disabled from the registry if that works for you.
Another option is to submit a similar post in Cloud flows forum and see if the way of writing the URL can be changed (eg; Url encoding etc).