There’s a section in your Partner Dashboard where you can provide your ABN details:
After hitting submit, you should see a green banner confirmation your information was submitted successfully.
You may also make these changes using your partner app by going to Account > Tax Info.
Please note, if you already have an ABN, this doesn’t necessarily mean you are registered for GST. You should contact the ATO or speak with a taxation professional for advice on whether you should or should not register for GST.