If you create a multi-select field in SFDC, let's say it called "Product of Interest," it syncs as a string field in Marketo. If multiple values have been selected in SFDC, they will be separated by a semicolon, e.g. Frosted Flakes;Cheerios;Wheaties.
If you use this field on a form, you will need to change the field type to Select to allow for multiple selections. If someone fills out the form and chooses Frosted Flakes and Cheerios, this will overwrite the existing values in the field and the field will now only say Frosted Flakes;Cheerios. These will resolve correctly in the SFDC multi-select field.
If you don’t want to overwrite existing data create a Marketo custom field for your forms. Call It “Product of Interest for Web Forms” and set it up on the form exactly as you would have set up the multi-select field.
Next create a smart campaign like the following in Marketo:
Smart List: Trigger, Data Values Changes, Attribute – Product of Interest for Web Forms
Flow: Change Data Value, Attribute – Product, New Value – {{lead.Product of Interest}};{{lead.Product of Interest for Web Forms}}.
Now, if you fill out the form and choose Frosted Flakes and Cheerios on the form, the "Product of Interest for Web Forms" field will say Frosted Flakes;Cheerios and the Product field will say Frosted Flakes;Wheaties;Cheerios. This will resolve to the correct values in the multi-select list in SFDC.
Additional resources for learning about the Marketo integration with SFDC.
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