how can I use IF ELSE in variables of azure DevOps yaml pipeline with variable group?

This code works.
I’m doing similar with parameters.

variables:
  - name: var1
    ${{ if eq(parameters.var1, 'custom') }}:
      value: $(var1.manual.custom)
    ${{ if ne(parameters.var1, 'custom') }}:
      value: ${{ parameters.var1 }}