The 22-year-old was initially named on the bench against Iceland, while team-mate George Hirst led the line in his first start for his country.
However, Slicker was brought on for his first senior appearance at international level in the seventh minute, replacing former Norwich City shot-stopper Angus Gunn, who picked up a knock.
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He had a poor start, conceding within a minute as Iceland picked up his short kick. Andri Gudjohnsen, son of former Premier League star Eidur Gudjohnsen, subsequently looped the ball over his head to make it 0-1.
Scotland responded in the 25th minute through John Souttar's strike, but fell behind just before the interval as Slicker failed to gather a corner. The ball bounced into the net off midfielder Lewis Ferguson, resulting in an own goal.
Iceland wrapped up the win shortly after the restart, with Victor Palsson's free-kick bouncing into the net off the gloves of Slicker.
George Hirst thought he scored his first senior international goal - but it was ruled out by VAR _(Image: PA)_
Hirst thought that he got his side back into the game soon after, but his first senior international goal was ruled out for offside by VAR in the 63rd minute. He was brought off for Che Adams five minutes later, having missed good chances to get on the scoresheet.
Elsewhere, Jack Taylor earned his first start for the Republic of Ireland, while Dara O'Shea was also named in the lineup for their friendly match against Senegal in Dublin.
Kasey McAteer put the Boys in Green ahead, but they were pegged back late on by Crystal Palace attacker Ismaila Sarr.
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Taylor played 58 minutes before being replaced by Evan Ferguson, while O'Shea played the full 90 minutes.
Finally, Nathan Broadhead was an unused substitute for Wales in their 3-0 win against Liechtenstein in World Cup qualifying.
Former Town striker Kieffer Moore wrapped up the victory after goals from Joe Rodon and Harry Wilson, sending the Dragons top of Group J ahead of their crucial clash against Belgium on Monday night.