Necroposting for nothing.
The OP is very poorly written and explained even worse.
Don't write bulls–t.
It's you who can't do it, not others. MikroTik can't fix your incompetence.
Open winbox and add it, EASY, or else LEARN how to do it with CLI/scripts.
[rex@example] /ip route vrf> add routing-mark=test
[rex@example] /ip route vrf> pri
Flags: X - disabled, I - inactive
0 routing-mark=test interfaces=""
[rex@example] /ip route vrf> set [find where routing-mark=test] interfaces=sfp9
[rex@example] /ip route vrf> pri
Flags: X - disabled, I - inactive
0 routing-mark=test interfaces=sfp9
sfp9 as expected
[rex@example] /ip route vrf> set [find where routing-mark=test] interfaces=sfp10
[rex@example] /ip route vrf> pri
Flags: X - disabled, I - inactive
0 routing-mark=test interfaces=sfp10
with SET sfp9 is replaced, not added, with sfp10 as expected
[rex@example] /ip route vrf> set [find where routing-mark=test] interfaces=sfp9,sfp10
[rex@example] /ip route vrf> pri
Flags: X - disabled, I - inactive
0 routing-mark=test interfaces=sfp9,sfp10
with SET sfp10 is replaced, not added, with sfp9 AND sfp10 as expected
[rex@example] /ip route vrf> set [find where routing-mark=test] interfaces=([get [find where routing-mark=test] interfaces],"sfp11")
[rex@example] /ip route vrf> pri
Flags: X - disabled, I - inactive
0 routing-mark=test interfaces=sfp9,sfp10,sfp11
this correctly add sfp11 with the other interfaces
work also if previous list is empty
Everything works as someone familiar with RouterOS would expect.
Obviously it should be checked before adding an interface if the interface is already there...
Obviously the script will be more complex if an interface needs to be removed from the list, rather than simply added...
but on winbox it's one click...
Obviously the obvious is obvious...