The standard way is the REST API Push endpoint: http://developers.marketo.com/rest-api/endpoint-reference/lead-database-endpoint-reference/#!/Leads/pushToMarketoUsingPO… You'll be adding leads to a Program, and the customer can take their own actions from there using Flow steps. You need to be very attentive to the client's API call limit, which is shared by all their integrations. The way to be a good integration "citizen" is to place an absolute limit on the number of calls you will allow per day.
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