In genealogy, it is best practice to record both where the original source is held and how you viewed it. The archive and call number identify the original; the Via field records where you accessed it.
Enter the access information in the Via field below the call number. You can type free text, or click the tag button to open a helper dialog with pre-populated drop-down lists for common access methods (e.g. FindMyPast, Ancestry). Click Build to generate the text including the date of access, then OK to apply it.