Why is :r SQLCMD command marked as wrong in Post Deployment Script?

I have worked a couple of times with post deployment scripts and always, intuitively used the build action "PostDeploy", because that is what it is. Now for the first time I try to follow the built-in instruction from the script's template to use the ":r somescript.sql" syntax. Immediately this line is getting marked as wrong: > "SQL80001 wrong syntax next to ':'" I found suggestions to set the PDS to Build Action "none". This does not help, the error stays. What am I missing here?