If the other parent with custody refuses to sign the application, you can ask the court for substitute consent for the application.
In the Netherlands, this is done by means of Article 1:253a of the Dutch Civil Code, which article can be used for all kinds of disputes between custody parents about the children. The request must be dealt with by the court within six weeks. If you can’t wait that long, you can start summary proceedings.
If several procedures are needed between parents to request substitute consent for issues concerning the children, the question arises whether it is in the children’s interest to have joint custody.