There are 2 ways to set priority for your Job Ads.
At the step 4 of Instant Post, you will be asked if you want to set priority to your job ads. If you have hesitation at that moment you can skip at that stage. And you can have second change to do it after your job has published.
If you had missed the first stage to set priority to your job ads, [ Job Listing ] will be your last chance to do so. You may click the [ Priority ] icon to activate the prioritize immediately.