Slack notifications: change channel ID property dynamically

Good day!

I'm using official slack notifier build feature and I wonder if there is an possibility to change "Channel or user ID" property via build step. I want to change it to slack username from git via "git log -1 --pretty=format:'%ae'" command.

Here is my build feature options:

It works well if env.userToNotify variable is predefined but if i try to do Write-Host "##teamcity[setParameter name='env.userToNotify' value='$slackName']" in my Powershell Build step new variable value isn't cached by build feature.

So the main question is how can I use updated variable value in slack build feature?