If the adhesive type is not of prime importance during the course of the study (not necessarily an orthodontic one), you can use cyanoacrylate adhesive or any epoxy adhesive.
If it is necessary to use orthodontic adhesive, you can follow the following steps steps:
1. Sandblast the metal bar with 100 microns or 150 microns aluminum oxide or do of grooving and pitting on metal bar to give some sort of mechanical retention.
2. Coat metal with one prime plus primer ormco. Follow the manufacturer instructions.
3. Use enlight adhesive ormco. Or any other orthodontic adhesive from any respectful company.