It is not the clearest thing in the docs, but you use the sample command that comes with MLTK and specifically use the partitions option (set to 10 is usually what you want) and then you have to search on partition_number < X. If you are doing the 70/30 split would be less than 7 as it starts counting at 0 and make sure to use seed option so you can come back and search partition_number > X-1 to get the other side of the split.
| sample partitions=10 seed=1234 | search partition_number < 7 | fit MLAlgoName target_field from whatever_fields into saved_model_name
| sample partitions=10 seed=1234 | search partition_number > 6 | apply saved_model_name as predicted_field
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