Initial Filter on a subpanel "Link an Existing"

You need to add the attributes relationship_file and relationship_class into target relationship and the define the relationship class accordingly, so you will be able to override the method 'buildSugarQueryRoleWhere' accordingly:

New attributes in the relationship

$dictionary["Account"]["relationships"]["account_cases"]["relationship_file"] = "custom/data/Relationships/CustomOne2MBeanRelationship.php";
$dictionary["Account"]["relationships"]["account_cases"]["relationship_class"] = "CustomOne2MBeanRelationship";

Changes in the relationship class, from the core relationship UserBasedRelationship

    protected function buildSugarQueryRoleWhere($sugar_query, $table = "", $ignore_role_filter = false)
    {
        $sugar_query = parent::buildSugarQueryRoleWhere($sugar_query, $table, $ignore_role_filter);
        
        $sugar_query->join[$table]->on()->equals($table.'.'.$this->userField,$GLOBALS['current_user']->id);

        return $sugar_query;
    }

I am sorry but I am not sure how your answer solves the problem of applying a dynamic filter to a subpanel. The relationship in question is a OOTB Many:Many relationship between Opportunites and Contacts.

Ok thanks to Ross and engineers at Faye, I have confirmed how to do this. Applying a custom filter when opening a drawer. - Dev Answers & Best Practices - DevClub - SugarClub (sugarcrm.com) explains it and give samples to help.