I would like to activate a tag in the PLC for 2 seconds and then have it deactivate automatically after pressing a button. Could anyone guide me on how to achieve this?

Use the pulse duration function in the I/O & Calc Tag - see help page below:

https://www.vtscada.com/help/Content/D_Tags/D_IOCalc.htm?Highlight=Pulse%20Duration

Just noticed you say discrete tag in the title, I'd prefer to handle this in the PLC (i.e. logic that sets the PLC tag back to the desired valve after 2 seconds. However, if you must do it in the SCADA you could use a set value button, then use a trigger tag to reset the tag after 2 seconds.