How to allocate a number according to a number range to a Z field in WEB UI?

Hi expert,

I created a Z field for a business transaction by AET in the web ui compoent.

This field is a field like transaction number, and we had created a number range for it.

But,I'm not clear how to determin the number for it during the business transaction creation?

For example,when open the business transaction creation page,calculate a number for it,lock it, and later, if cick "SAVE", assign the value to the field,if click cancel, release it.

Is there any suggestion how to achieve this?

If any sample coding,it will be greatly appreciately.

Thanks.

Jerry.