I assume the record does not exist of his birth. From Scotlands People, the closest candidate would be 02/06/1736 Andrew Forrest son of Andrew Forrest and Christian Taylor of Canongate Edinburgh City. An Andrew Forrest b. circa 1707 of Edinburgh (son of Patrick Forrest) married Christian Taylor 14 May 1732 at Edinburgh. However I have no reason to suppose that the family were in Edinburgh and not Dalkeith or surrounds at that time.
The only, and probably correct, candidate for Jean Johnston's birth is 18 15/09/1745 Jean Johnston daughter of James Johnston and Jean Watt of Newbattle.
Scotland People give the banns for both bride and groom
08/12/1761 Andrew Forrest and Jean Johnston banns read at his parish Dalkeith,
Midlothian
14/11/1761 Andrew Forrest and Jean Johnston banns read at her parish Newbattle,
Midlothian
They were married on 8th Dec 1761
The cautioner for the man is James Forrest (I would assume is the man that marries 13 Jun 1760 to Margaret Mabine at Dalkeith)
Born to Andrew and Jean at Dalkeith are:-
Helen Forrest 19 September 1762 Dalkeith, Midlothian, Scotland
Isobel Forrest 20 March 1765 Dalkeith, Midlothian, Scotland
Margaret Forrest 8 January 1767 Dalkeith, Midlothian, Scotland