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See default field values to understand the precedence that XML documents sent in the Post request to form B take over the other possible methods of defaulting form field values.

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Using Templates

Fields in form B can also be initialized with data from form A using Url Parameters. Imagine form A contains a from palette text control labeled 'Customer Name' with Name property set to 'custName'. And form B has a palette text control labeled 'Full Name' with Name properity set to 'fname'. Using either Form Action Go to Url or Post with the URL set to that of Form B, append &_data=(fname:'{custName}') and when form B loads it's 'Full Name' field will be set to the value entered into form A's 'Customer Name' field.

See initializing via _data Url Parameters in the Initializing Forms with Data section for more details on using _data.Because you many not be in control of what a user enters into a form field used in this way, you should always quote template variables. The only time it is not needed is for numbers but even then it will work with quotes. For example, in the Form Action URL the template variable should have '{name}' quoted as follows: _data=(Name:'{name}').