I too need to save multiple values from a picklist into a field. The field in question is expecting a string but i can't find a definition for the format of the string (csv, xml etc).
If it is a multi-value picklist, the way you set the value is to submit a string with semi-colon delimited values. For example, if the picklist values are:
AA BB CC DD
Then setting the field to: "AA;BB" gives you those two values.
If it is a multi-value picklist, the way you set the value is to submit a string with semi-colon delimited values. For example, if the picklist values are:
AA
BB
CC
DD
Then setting the field to: "AA;BB" gives you those two values.