You knit the strap starting at the back and when you get to the front, three sts go on a holder and the outer (armhole) 3 sts start the I-ccord bind off. You continue the I-cord along the underarm, behind the back strap. So you don't pick up sts where the strap began, you work plain old I-cord behind those strap sts. Then you continue with the I-cord bind off across the back of the tank to the other, unworked back strap.
When you start the new end of yarn, be sure to leave about a 6 inch tail to weave in later. With that size tail, the end won't come undone and weaving in the end will secure it.