I want to set in maintain purchase order lines on form 3

activity as = P01 ,Act_def,
element = 09

this is by default i want to keep for all future creating order.

how and where to set ,which sessions.

Please advice.

You can edit this form in the Maintain Forms {ttadv3100m000} and copy this form to your own VRC. Save the form and open the editor by clicking on Edit Form button and open the property setting for the field in question. In the property window on the right bottom side you can see the field "Default Value". Enter the value you want to put in double quotes (if string). Save and exit the editor.
Save the form and click on the button Dump.

Another way of setting the default is by going to the Form fields (i.e if you know the field name you want to set a default for.). In Form 3 you can set the Default Expression

sk

P.S: For some form types, you might have to bring the cursor to the field(i.e Form 3 of the purchase order line in your case) to make the default value appear.