You simply add a class having z-index as an attribute having a model header.
In order for the z-index to work, you also need to set position = relative, absolute or fixed. A z-index of around 5000 can also help. (A modal may have a z index above the 2000s.
so in your css I would add the following:
.class-of-dropdown { position: relative; z-index: 5000; }
The .modal-body class has the overflow-y: auto property. You may need to change this to:
.modal-body { overflow-y:visible; }
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