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You're going to need a developer if you want to expose the Marketo REST API in any way other than direct connection (for example, if their system is not capable of hitting the OAuth /identity and /push endpoints for a lead push, and they need a single-request JSON interface to your system, you have to build an intermediate gateway where you do the /identity bit for them).
And unless their system already has a published endpoint that's webhook-compatible for you to hit, you'll need a developer for that, too.
There's no one-size-fits-all way to do this, because it's going to be determined by the data structures + transfer methods in use on both sides.
At its simplest, both sides already support a form-compatible endpoint (that'll also be naturally webhook-compatible as well). At its most complex, both sides have a robust REST authentication + update infrastructure, so each side needs to build to the other side's needs.