How was 言 created by adding one mark to 舌? To create 言 - Wiktionary, mustn't you add more than one mark to 舌? Sorry for not being able to visualize abstractly!

Ideogram (指事): 言 was created by adding a mark to 舌 to indicate movement of the tongue.[1] This indicates speech where 舌 is a mouth (口) with a tongue sticking out.
The description

言 was created by adding a mark to 舌

should really be

言 was created by adding one or two horizontal marks to the top of 舌

Details aside, it is a fairly accurate character component arrangement description. For it to make sense, you have to first determine the shape of the character components that it's talking about.
Wiktionary's glyph evolution table of 「舌」 doesn't show enough examples of 「舌」 in oracle bone script for the description to immediately make sense. The shape we have today is originally inherited from Series 1, shown below:
Series 1


Crossed out samples are ones where spittle is also drawn, which doesn't survive today.

The confusion arises because Wiktionary is only showing a sample of 「舌」 from Series 2 below:
Series 2

It doesn't really matter which shape of 「舌」 that 「言」 is derived from, 「言」 is still made up from one or two horizontal marks added to the top of 「舌」:

「言」 derived from Series 1 「舌」:
商甲甲499合集30697

「言」 derived from Series 2 「舌」:
商甲拾8.1合集21082商甲乙766合集22072


You can even get 「言」 derived from 「舌」 with spittle drawn:
商甲後2.41.9合集21631

Glyph origins summary
商甲甲499合集30697戰國金中山王方壺集成9735秦簡秦律十八1睡虎地秦簡西漢隸老子・甲馬王堆帛書楷　
「言」 (to speak) was originally comprised from one or two horizontal marks 「一・二」 added to the top of 「舌」 (tongue). The shape which survives today is made up from two horizontal marks 「二」 on top of a 「舌」 shape coming from Series 1.
「舌」 is no longer evident in 「言」 due to some minor shape corruptions to the forked tongue shape occurring post-Qin. Compare 「音」, which is actually comprised from 「言」 with a differentiating mark added inside of the 「口」 part of 「言」.

References:

季旭昇《說文新證》
小學堂
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》