Use o Powershell para escrever um script que pressione Ctrl + Shift + F após iniciar o Slack. Por exemplo:
& ~\desktop\Slack.lnk # launch slack
$wshell = New-Object -ComObject wscript.shell; # initialize wscript
$wshell.AppActivate('Slack - My Workspace') # change this to the title of your slack window; activate it to receive the key combination
Sleep 10 # wait for it to open
$wshell.SendKeys('^+{F}') # press the Ctrl+Shift+F key combination
Referências