For Juniper Networks EX/QFX Series that are managed by Juniper Mist, you can use the Modify Virtual Chassis option on the switch details page to manage your Virtual Chassis.

 

The Modify Virtual Chassis workflow leverages the pre-provisioned way of Junos configuration. This option is visible for switches that have the configuration management option enabled in Mist and for super admin users. Please note that in order to use this feature, you cannot have additional CLI commands specific for virtual chassis as they will take precedence over the GUI-defined ones.

The preprovisioned configuration specifies the chassis serial number, member ID, and role for both member switches in the Virtual Chassis. When a new member router joins the Virtual Chassis, Junos compares its serial number against the values specified in the preprovisioned configuration. Preprovisioning prevents any accidental role assignment to a Routing Engine, or any accidental addition of a new member to the Virtual Chassis. Role assignments, member ID assignments, and additions or deletions of members in Virtual Chassis are under the control of a preprovisioned configuration.

This workflow applies to all the EX Series and QFX Series platforms that support Virtual Chassis:
https://www.juniper.net/documentation/us/en/software/mist/mist-wired/topics/topic-map/modify-vc-preprovisioned.html#:~:text=the%20Virtual%20Chassis.-,On%20the%20Mist%20portal%2C%20navigate%20to%20the%20switch%20details%20page,page%2C%20click%20Modify%20Virtual%20Chassis.