On your posted reel, click the lines in the bottom right corner and in the menu that pops up, select "Manage".
Select "Partnership label & ads" and toggle ON "Get partnership ad code". Copy the code and paste it in the designated field in the reel submission form.
If you do NOT see the "Partnership label & ads" option, go to your Instagram account settings and go to "Account type and tools" > "Branded content" and ensure your account is eligible and active.
Choosing a selection results in a full page refresh.