Every time you add a listing, there is a unique link generated for showings. You can find this unique link under the links tab of a listing. Copy and paste that into your MLS comments and let agents know they can view the property by following the link. By default when the agent fills out the online form, the lockbox code will display for them on the screen.
You can also require the showing to be scheduled by checking that option on the lockbox tab. This will send you a notification to your email and let you approve of the showing before giving out the lockbox code.