7 – Multiple Choice Entity Reference

I have an entityreference field on the users and I can not find a way to select more than one value. I want to do as a multiple selection, but with the entityreference.

I'm using the EntityReference module, here is my form element:

$ form['maj_planning']['suivi_par']    = table (
& # 39; # type & # 39; => & # 39; entityreference & # 39 ;,
& # 39; # title & # 39; => t (& # 39; Tracked by & # 39;),
& # 39; # required & # 39; => FALSE,
& # 39; # default_value & # 39; => $ entity-> tracking_par,
& # 39; # era_entity_type & # 39; => & # 39 ;,
& # 39; # era_bundles & # 39; => array (),
& # 39; # era_cardinality & # 39; => 1,
& # 39; # era_query_settings & # 39; => table (
& # 39; limit & # 39; => 15, // the default value is 50.
& # 39; property_conditions & # 39; => array (),
& # 39; field_conditions & # 39; => array (),
)
)

How can I achieve this goal?