On the email Settings tab of Create (or Edit) email Template in Acoustic Campaign it is possible to select Share template with CRM system. This can be used to limit the templates that you allow for selection within Salesforce. When a email template is marked as a CRM template, the creator can enable a CRM Block, that is, an area in the email body where the sender can add text at the send time that could be inserted into the body of the template.

In Acoustic Campaign, place %%SP_CRM_BLOCK%% in the spot where the custom text should appear in the body of your email Template. The GetemailTemplates response indicates whether the template contains %%SP_CRM_BLOCK%% based on the ALLOW_CRM_BLOCK element. If true, you could allow specifying text in your application's user interface and pass it to the Scheduleemail operation.