You capture the value into a variable first. Then go to the second table's layout, create the new record. set the field, and then return to the orig layout.
Vodka, that cover the second item I take it?
If you have a relationship that allows creation of child records, and a portal on the parent layout, you can create a new child record by going to the last portal row and setting fields there.
However, there's no good reason for a script to imitate what a user would have done, and become dependent on the relationship setings and the presence of a portal.