[Freesurfer] inter-hemisphere registration on customised template

Dear Freesurfer experts,

I am writing to ask how to do the surface-based inter-hemispheric registration based on the customised template, instead of fsaverage.

I am planning to calculate the lateralisation index for each vertex, which means I should to use the symmetry template, for example, fsaverage_sym. However, the current issue is my participants are children. Therefore it would be better to put the individual data to the customised average template, which is already created by ANTS in my case. So I was wondering how to perform the symmetry registration on this created template?

Thank you in advance.

Cheers, Ting