typically you just need to have a text column, and the textarea input in your form will be replaced by tinymce if you initial and setup according to the documentation correctly... at the end, system will just treat the HTML content as an input value and you should be able to save it as like other input...
dd() the input parameter once received to double check the value is received, one of the common mistake usually is forget to give the id and name for input control...
Thank your for this Answer @siangboon, but im also tyring to access the value with javascript but this is giving me null => var text = document.getElementById("Grundtext").value;