Explanation
If I knew her name, I would tell you means
اگر مجھے اس کا نام معلوم ہوتا تو میں آپ کو بتاتا
The second part of the sentence is in the past tense.
The verb to be used should also be in the same tense.
The past tense of the word "know" is "knew".
Therefore, the correct tense here is past tense and the verb used is "knew".