The FL403 is not the correct route.
An interim order has been granted without you being present, and without the court hearing any evidence except reading what is in your ex's application and statement and making no findings at all at that stage, but deciding the order was needed until they could investigate further and hear from you.
You task, therefore, is to respond to the application and the statement your ex made in full. You need to copy the format of her statement and change the relevant parts (e.g. first statement of YOUR NAME), and then one by one respond to each and every one of the allegations she raised against you in her statement and deny them, dispute them, and or put your own version of events for that issue. You need to do this carefully and one by one so that you cover everything she has said. If you have any evidence to disprove what she has said you need to attach it to your statement as exhibits.
If the court decides that it needs a further hearing because it cannot tell which of you is telling the truth, it can set a date for a fact finding hearing where each of you will be called to give evidence, and cross examined - and where you are likely to be able to ask if you have other witnesses that you would like to do statements and appear in court where they have been witness to each other's alleged behaviours. If the court wants a fact finding hearing, the interim order will remain in place until that hearing when further decisions about it will be made.
Alternatively, if the court doesn't believe a fact finding hearing is necessary, they may make a decision about the NMO on the basis of the statements and having heard from both of you. There is the possibility that an undertaking could be issued instead - this is a promise to the court that you won't do what you are accused of, but which the court clearly identifies is given on the basis of no admittance from you. This is often the better outcome for people who share children, but should be given cautiously, particularly if you don't accept that you have done anything wrong and it's not needed. Whilst it is lesser than an NMO, you can still, ultimately, be arrested if you breach it.
Having an undertaking or an NMO does not stop you from making an application to have contact with your children.
To clarify - you do NOT fill in an FL403, it is unnecessary as the application made is still before the court - an FL403 is for an order that is in place and has been for a while. Do a witness statement with evidence/exhibits as advised above.