Limit options for a field based on value of other field

Content

We have two fields in Incident Workspace.
1. Organization - Custom Object
2. Entity - Custom Object (currently this is search report field)

I would need to restrict the entity field options based on what is selected in Organization.
In BUI extension, I would prefer to make Entity field as select tag (html select element), so the options could be restricted.

If I add a new select tag (field) using BUI extension, how to save this field value, whenever the incident is saved.