Fast Bundle offers "Bundle builder" for some of the bundle types like "Collection Mix and Match", "Product Mix and Match" and "Buy X Get Y" bundles.
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You need to put this code as a custom liquid where you want to place the bundle builder.
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Instead of the "XXXXXX", you should put the "Bundle ID".(Available on the Fast Bundle admin for that bundle in More displays)

So the code will be something like this:
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