This article acts as a workaround for allowing Portal users (Customers) to change the issue Priority using "Jira Forms" (I highly recommend you to give the documentation a good read if you are not familiar with Forms, so you can see how awesome this feature is!) since at the moment isn't possible to do it natively.
Being said that, let's see the steps to achieve it:
1) First we need to create a Form (Go to Project Settings > Forms) and click on Create Form, this will lead us to the "Form builder" page where the Form is designed and configured. For this use case, we will use a "Dropdown" field and linking it to the "Priority" Jira field, e.g.
Then click on the Settings tab, we would like to leave the Form Open so Customers can modify it upon issue creation, so make sure to check the "Keep form open so that anyone can edit and submit it after the issue is created" option:
Note: Make sure to Save the changes.
2) Once we've got our Form created. Next step is to create an Automation rule that whenever an issue is created, will attach the Jira Form to it allowing to see the "Priority field under the Customer portal issue view.
Go to Project Settings > Automation and click on Create rule. As a trigger choose "Issue created" and after that add the "Attach Forms" action:
Under the Attach Forms configuration make sure to add the Form created on step #1 and leave the Form visibility as External:
After that make sure to save and Publish the rule.
That's it, once an issue is created via Portal, the Form will get attached and will be visible under the issue view (You may need to refresh the page in order to see it, since it takes a little bit to load upon issue creation).
Here's a brief demo of how this will work:
I would like to mention there are some caveats while doing this approach:
I hope this helps :)
Rogelio Martínez | Atlassian Cloud Support