MongoDB partial index optimization

I have a MongoDB collection and the following query:

{  isActive: true,  managerId: null,  clientId: { $ne: null }}

I would like to optimize this query by creating an index. It looks like partial index would be a good solution:

createIndex(  { /** @todo: specify keys */ },  {    partialFilterExpression: {      isActive: true,      managerId: null,      clientId: {        $type: 'string',      }    }  })

Now I know what to put in the partialFilterExpression, but I don't quite figure out what fields should I put in the "keys" property.

My query has a fixed constant values everywhere, there is no variable parameters.

What would be the most optimal index for this kind of query?