Single Input View Form - PHP & JS

Hello: I am new here, also new coding with PHP & js. I have almost finished a "single input view form" but when the form is submitted the success message is showing on another page (a blank page). I would like that the messages will show on the same page where the form is. I hope you can help me....Let me explain in detail:

HTML:

Submit Request Continue

stepsForm.js:

var theForm = document.getElementById( 'sc_contact_simform' ); new stepsForm( sc_contact_simform, { onSubmit : function( form ) { // hide form classie.addClass( theForm.querySelector( '.simform-inner' ), 'hide' );

                form.submit()
            
            }
        } );

include/contact-form.php: