Under Settings > Inbox Automation, you'll find Inbox Rules. From here, you can create a new rule to prioritise a conversation if it contains certain keywords.

When creating a new rule, select the event "A customer replies to outbound content".

Then, in the conditions for your rule, select Message Content, and define the keywords you want this rule to target.

Then, choose the action Mark as priority. Save your rule, and you're good to go!