How do I add a vocabulary to a metadata in the item's metadata edit form in DSpace 6.x xmlui

I can add a vocabulary to a metadata in the submission form while creating an item. However, when I try to edit the item, I do not see the vocabulary to select the value from. I could not find any documentation relating to customizing the edit form for an item. Can anyone point me to the right direction? I am using version 6.x of DSpace.

Screenshot of edit item form

Screenshot after enabling authority control

You can only see the lookup button beside that metadata in the submission form because that is for the submission process only. You have to tell DSpace that that metadata is authority controlled. Please see this link: Authority Control of Metadata Values

Assuming we will use srsc as the controlled vocabulary, your settings should look like below. Note the last line, e.g. authority.controlled.dc.subject= true, that tells DSpace that the dc.subject field is authority controlled. You can find this settings in dspace.cfg under the ##### Authority Control Settings ##### header but you can override it by putting it in local.cfg.

## demo: subject code autocomplete, using srsc as authority
## (DSpaceControlledVocabulary plugin must be enabled)
## Warning: when enabling this feature any controlled vocabulary configuration in the input-forms.xml for the metadata field will be overridden.
choices.plugin.dc.subject = srsc
choices.presentation.dc.subject = lookup
vocabulary.plugin.srsc.hierarchy.store = true
vocabulary.plugin.srsc.hierarchy.suggest = true
vocabulary.plugin.srsc.delimiter = "::"
authority.controlled.dc.subject= true    

Also, make sure that DSpace Controlled vocabulary is enabled, i.e., webui.controlledvocabulary.enable = true and:

plugin.selfnamed.org.dspace.content.authority.ChoiceAuthority = \
   org.dspace.content.authority.DCInputAuthority, \
   org.dspace.content.authority.DSpaceControlledVocabulary

exists.