Make it prime with the smallest suffix
Jelly, 7 bytes
1ṭVẒɗ1#

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How it works
1ṭVẒɗ1# - Main link. Takes n on the left
    ɗ   - Define a dyad f(k, n) from the previous 3 links:
 ṭ      -   Tack; Yield [n, k]
  V     -   Eval; Smash together into a single integer
   Ẓ    -   Is prime?
1    1# - Starting from k = 1, find the first k such that f(k, n) is true