Fill a entry form with a script

Due to law changes in EU, we need to send an Opt-In message to Newsletter Customers of one of our clients. There for we have a form which needs to be filled automatically with data. This function needs to be created in a way that we can use it on our own. You will just get test data to set up the script, later we need to be able to upload correct data on our own. The form is integrated inside a Wordpress Website with Ninja Forms ( [login to view URL] ). This form is connected with Clever Reach which is a Newsletter software. For further information please contact us.