hello

when you write into the many2many field, at that time you can use like 

product_id.write({'route_ids': [(6, 0, list_of_route_record_ids)]})

for write in field many2many type the process is same, just using widget it display data in different format.

so override the default_get method and in the return result of the method you have to update the value.

@api.model
def default_get(self, fieldslist):
    res = super(classname, self).default_get(fieldlist) 
    res.update({'route_ids': [(6, 0, record_ids)]}) # where record_ids = you have to search route ids and pass the list of record
    return res