If you create a sample meta field that is defined in the sample type, make sure that you specify the correct sampleTypeMetaID for it. In addition, the sampleDataType should be the same as defined in the sample type.

You can create a meta field that isn't defined by leaving the sampleTypeMetaID blank. This is not advisable though, and results in the meta field's value not to be searchable!

In case of a meta field with selectable value - data types CHECKBOX, COMBO and RADIO - make sure that you only specify the values that are listed in the sample type's optionValues property. In case of multiple selected values, separate each value with a comma.

Note: it's advisable to use the PUT variant of this call. The POST variant will always make a new field, even if one already exists.