How can I limit the options under the 'Link Assets' dropdown on the Support Portal?

[Rafiur R. Bhuiyan]

Hi, 

I wish to ensure the following:

When an authorized user will visit our support portal and try to raise a request via e-mail, they will be allowed to link ONLY the assets that have been assigned to them. How may I enable this? 

Kindly let me know.

Thank you,
--
Rafiur