OED has 1930 for "Wait for it":
Wait for it
said (often parenthetically) to create an interval of suspense before imparting something remarkable or amusing, in order to heighten its effect. Also ironically. colloquial.
1930 M. Allingham Myst. Mile xviii. 170 ‘Wait a minute,’ said Mr. Knapp. ‘Wait for it... That is just exactly wot I do know.’
1966 ‘H. Calvin’ Italian Gadget ii. 21 We can have a shower and..wait for it, dinner at the Palazzo Capucci.
OED doesn't have the exclamatory encouraging usage of "got this" at all, although it might be derived from
10. a. transitive. To win (a victory), to gain the favourable issue of (a battle, a field of combat, a match, etc.).
c1330 Horn Child l. 462 in J. Hall King Horn (1901) 184 At iustes & at turnament..euer þai gat þe gre.
[Get is so old and the OED entry is so correspondingly long that I may have missed a more appropriate instance, though.]