PowerApp.Patch.Multi.MMD.js

// First of all we need to loop through each of the selected items in the combo box...
ForAll(
     // Where 'MyComboMultiSelectBox' is your multi select combox box containing the selected items...
     MyComboMultiSelectBox.SelectedItems,
     Collect(
             // Create a new collection called TaxonomyValuesFromComboBox
             TaxonomyValuesFromComboBox,
             {
                // Create a new entry in the collection for each of the selected items...
                '@odata.type':"#Microsoft.Azure.Connectors.SharePoint.SPListExpandedTaxonomy",
                TermGuid: TermGuid,
                WssId: WssId,
                Label: Label,
                Path: Path,
                Value: Value
             }
     )
);

Patch('My Document Library Name', {
  // Save this collection against the internal SP Column name...
	MyInternalMultiTaxonomyFieldName: TaxonomyValuesFromComboBox
});