linux - Assign IPv6 subnet to interface - outgoing requests

I received a IPv6 subnet /64 from my provider (vultr). Many people are asking how to assign whole subnet to interface. As far as I understand, the easiest option is ip addr add 2001:4860:4860:abcd::/64 dev eth0. This works fine for incoming requests to any ip in subnet. But how can i use them for outgoing request? With this command my outgoing ip is 2001:4860:4860:abcd::, but i want to use any in range, like 2001:4860:4860:abcd:c37:a324:40f3:98fd.
Is it even possible? Or does this relate directly to the program that makes the requests?