Hi David,
thank you for your question.
If the block-view consisting of ner_manual and text_input is sufficient for you, you could do a simple post-processing step where you add the "user_input" field as the span's label. This could be done for example by implementing a before_db method such that the span label is modified before it is saved in the database (https://prodi.gy/docs/custom-recipes#before_db). But be careful here since a small bug in this function can lead to data loss.
Otherwise, if you want to add the span label on the fly, you might find this answer by Ines interesting even though this might need a bit more effort.