You can use auto margins

Prior to alignment via justify-content and align-self, any positive free space is distributed to auto margins in that dimension.

So you can use one of these (or both):

p { margin-bottom: auto; } /* Push following elements to the bottom */
a { margin-top: auto; } /* Push it and following elements to the bottom */
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Alternatively, you can make the element before the a grow to fill the available space:

p { flex-grow: 1; } /* Grow to fill available space */