Value separated by comma cannot be stored in custom field

CF.{Line no.}: 7,8,9Above line is contained in the post data to create a new ticket via REST API. And custom field named “Line no.” has type “Enter one value”. However, value 7,8,9 is not stored in the field. Is there any special rule for this kind of field? And what format I should follow to format the value? Thanks!
Chenxiong Qi