I think what you’re looking for needs to be done from PHP, editing the subpaneldefs.php file.
Try looking for examples online, or even for an existing subpanel in SuiteCRM that does an equivalent thing. Something that shows a field from a related module.
For example, in Contacts detail view, the Cases subpanel references an Account name.
This comes from the file modules/Contacts/metadata/subpanels/ForCases.php, in this part:
'module' => 'Accounts',
'target_record_key' => 'account_id',
'target_module' => 'Accounts',
'widget_class' => 'SubPanelDetailViewLink',
'vname' => 'LBL_LIST_ACCOUNT_NAME',
'width' => '22%',