Set customfield
This post function will set a new value to a custom field(s) of an issue in a transition.
Fields
Choose a field: This is a select field where you be able to select a custom field from the current instance, when a field is selected a new input field will appear depending of the type of the field selected.
Add field button: Click it to add a new empty field.
Request size limit
In Jira Cloud there is a limit on the size of the requests. If the limit is reached then a message will popup when you try to update/add the post function.