I am trying to inject a variable from a file which will tell a Jenkins test job what perforce label it should use during SCM

My file is named change.properties and looks like this:
p4.changelist=386494

In the test job, I have clicked the 'Prepare an environment for the job' section and then added the file location in the 'Properties File Path' box

In the Perforce section, I am setting the perforce Label to be GPTCDTEST${p4.changelist}

However when I run the job, I get this error:
Invalid changelist/client/label/date '@GPTCDTEST${p4.changelist}'.

which makes me think that it wasn't able to pick up the p4.changelist variable I injected.

I have been able to do this by having an upstream job trigger the test job with parameters from the file, and this works fine, but I want to be able to have the job run on it own without requiring the upstream job.

I also tried just inserting this variable in the 'Properties Content' section, but Perforce still can't find it:

[EnvInject] - Injecting as environment variables the properties content
p4.changelist=322332

[EnvInject] - Variables injected successfully.

...

com.tek42.perforce.PerforceException: Errors encountered while force syncing: error: Invalid changelist/client/label/date '@GPTCDTEST${p4.changelist}'.

Is there an issue with linking up a variable from the Prepare Environment section to the perforce SCM section?