InDesign's TextFramePreference object exposes a key, firstBaselineOffset, which reflects the options available in the Baseline Options tab. In the Offset dropdown list, the user can choose among Ascent, Cap Height, Leading, x Height and Fixed. There is no other choice in this dialog:

Available modes in the First Baseline Offset list.

But when you inspect the possible values of the FirstBaseline enumerator, an extra option appears: EMBOX_HEIGHT.

The Scripting model offers a hidden option: EMBOX HEIGHT.

For some reason, this baseline offset mode is not visible in the UI although available in the scripting object model and fully functional in InDesign for a long time.

To assign the Em-Box offset to the first baseline of a text frame, just use the following code:

// Assuming a text frame is selected.
var tf = app.selection[0];
const EMBOX = FirstBaseline.EMBOX_HEIGHT;
tf.textFramePreferences.firstBaselineOffset = EMBOX;