Hi Junminya,
You will have to do it by webscript FTL .You can not do it by share-config-custom as "Customize Dashboard" link is not configured in share-config file, else you could have done by adding "condition" in link.
You need to extend/modify - user-dashboard-title.get.html.ftl
<javascript>
<#if user.isAdmin >
<a href="${url.context}/page/customise-user-dashboard" ${linkClass}>${msg("link.customiseDashboard")}</a>
</#if>
</javascript>