Your first suggestion looks fine. It doesn't seem likely to be misconstrued. There are other similar sentences that would convey the same information in slightly different words, such as:
This tag is assigned if FieldName is Value
This tag is assigned when field FieldName is Value
This tag is assigned if field: FieldName is Value
This tag is assigned if ...
I would argue against the use of "is" to describe the relationship. There are too many usages of "is" to allow it to be interpreted with the precision that technical writing must have.
This is overly fussy, but after decades of trying to communicate technical details to non-technical audiences, I feel totally justified.
For an exact match,...