You can achieve this thing using overriding the create method in odoo.

def create(self, cr, uid, vals, context={}):

     if vals.get('project_id'):

         project_obj  = self.pool.get('project.project').browse(cr, uid, vals.get('project_id'), context=context)

         task_sequence = (self.pool.get('ir.sequence').get(

cr, uid, 'project.task.new', context=context) or '/' )+ (str(project_obj.SEQUENCE_FIELD_name)  or '')

         vals.update({'TASK SEQENCE FIELD': task_sequence})

     return  super(CLASS NAME, self).create(cr, uid, vals=vals, context=context)

Hope this will help you.