What user action do you want to be the "trigger" for that script to start executing?
If you want it to start executing when the user clicks it, then select the field while in layout mode and use Button Setup.. to turn the field into a button.
If you want it to start executing when:
The field is exited
The field is entered
A key is pressed when the cursor is in this field
The field's contents are changed
The field is "saved"
You'd use a Script Trigger. Right click the field in layout mode and select "script triggers"...
You can learn more about triggers in the filemaker help system.