Just put the hashtags in a line at the end with a blank line above them.

You don't need to change any settings. If a reader's app or interface is compatible with displaying them in grey boxes, it will do so automatically as long as they're at the end with a blank line before them.

I've written some examples of tags below, but set my post to unlisted visibility so I'm not spamming them 🙂

#ForExample #TheseTags #AreIn #GreyBoxes #OnMany #Platforms