A border tax advantages US exporters while presumabuly minimizes or eliminates deductions from imports. Ths puts a premium on US generated goods and services, putting a resultant premium on the USD at a time when the hawkish fed is reasiing rates - putting additional upward pressure on the dollar.
This combination, may not effect US companies in the short term (as they will get tax subisdies to offset the rise in the dollar, which may not even rise at the same rate that the subsidies are given) but commodity and rate sensitive nations, ie. OPEC nations, can feel the burn. There have been rumors of Saudi needing to depeg from the dollar for some time now, due to their economy growing much slower than the US. As compiled by Bloomberg:
Their alternamtive may be to cut production, which they've promised to do multiple times, but truth be told... They're not in the position to do that for long.