Customfields: Fill in a dropdown menu field via API (core_course_update_courses)

Dear Community,

We would like to use the customfields (type: dropdown menu field) and wan't to fill in values via webservice call (core_course_update_courses). The customfields and values will be created by us in advance, eg:
fieldname: category
dropdown menu entrys:

• Winter term 2019
• Summer term 2020
• Winter term 2021

...


Arguments:

customfields  Optional //Custom fields

list of ( 

object {

shortname string   //The shortname of the custom field

value string 

Does anyone know if it is possible to fill in values in a field of type dropdown menu?
What's the value string? Is it eg "Winter termin 2019"?

thanks, best regards
Anton