Here is a working example with a user profile migration (not Drupal user account).
Adress is an array of subfields. You can check in database all the possible subfields because i don’t use all of them (ex : field_profile_address_family_name)
.yml file :
field_profile_address/country_code: plugin: default_value default_value: FR field_profile_address/langcode: plugin: default_value default_value: fr field_profile_address/address_line1: adresse1 field_profile_address/address_line1: adresse2 field_profile_address/locality: ville field_profile_address/postal_code: code_postal