Issue linking can be done from the Issue's details page under More Actions button. You will se an option called Link. However, if you require a separated button for Linking issues, then I'm afraid that this is not possible from the UI level, you need to go to JIRA code and customize it. For further information about Linking Issues, please refer to the following link:
'Relates to' (the link type) describes simply the relation between two linked issues.
For example you got two issues 1 and 2, now you found out that they have a common root cause. You would link them using the 'relates to' link type.
If they are the same issue you would link them as duplicates.
if you can fix issue 1 only after you fixed issue 2 you would link them as 'blocks' (respectively as 'is blocked by')
You can even define your own link types if you want
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