I managed to make it work somehow:
async find(ctx) {
await this.validateQuery(ctx);
let sanitizedQueryParams = await this.sanitizeQuery(ctx);
sanitizedQueryParams.filters.$and = [...sanitizedQueryParams.filters.$and, { user: { id: ctx.state.user.id } }]
const { results, pagination } = await strapi.service('api::yourservicename').find(sanitizedQueryParams);
const sanitizedResults = await this.sanitizeOutput(results, ctx);
return this.transformResponse(sanitizedResults, { pagination });
},