If you pre-ordered that game. Eventually, you will get this pre-order Bonus — but for that you need to do some grinding in-game. It won’t be waiting for you at Postmaster or in you Vault. To obtain Coldheart Exotic Rifle:
Players must complete whole story campaign. This means 17 missions in total which is over 16-20 hours of gameplay!
After that, you need to be Level 20 that is the Max level in Destiny 2 at this moment. You will get to Level 20 after completing the story campaign — but if you don’t, you need to grind your way up to level 20 by doing Nightfall, Strikes, Public Events or play some Crucible.
After completing the campaign and reaching level 20 — Your Pre-order Bonus will be waiting for you at Gunsmith.