Custom fields is available only for Essentials and Enterprise+ tiers. Only Admins and those who've edit permissions on a plan can apply custom field template to that plan.
Sometimes you need to be able to collect information about your plans beyond just timelines and ownership - that's where custom fields can come in. Once you've configured some custom fields and saved them as templates - see Create Templates and Custom Fields, you can then start applying these templates to your plans to give you scalability and unlock some strategic insights that're more relevant to your organization.
Go to the desired plan, and in the Planner page, click the three dots against the Plan name, and select Plan settings.
Expand the Custom Field Template field, select the desired template from the list and click Save.
You can add only one custom field template to a plan. However, the same custom field template can be applied to many plans. If you've different custom fields created in different templates but want all those to be applied to a single plan, then create all those relevant custom fields in one template and then select that template to the relevant plan.
This will apply the custom fields defined on that template to the corresponding entities on the plan. You can remove or update the template applied to a plan at any time from Plan settings. Let's see how to populate and visualize data from custom fields in Populate Data against Custom Fields article.