The unscripted method is to use "same as" for the operator.

If you want a scripted method, check the advanced button and use the script field with a line of code (in place of the comment) like this:

current.work_start = current.u_other_date_time_field;

If you want to set it to the CURRENT date:

current.work_start = gs.nowDateTime();

GlideSystem Date and Time Functions - ServiceNow Wiki