Short answer: There is no universal, scriptural rule that a shravaka must wear gandha (fragrance paste) on the forehead during swadhyaya. Swadhyaya is primarily about study and contemplation on Jain scriptures; marks or adornments are not a required part of the practice.

What you may keep in mind:

• In Jain practice, inner purity and right understanding (samyak darshan, jnana, charitra) are the focus during swadhyaya.
• Some households or communities may use a mild sandalwood paste or tilak as a local custom or as a respectful mark during worship or study, but this is not a core Jain requirement and is not uniformly followed by Digambar or Śvetāmbar communities.
• Digambar monks renounce such adornments; lay followers may adopt simple, unobtrusive practices, but again, there is no canonical mandate to apply gandha specifically for swadhyaya.

If you want, I can point you to a JainKnowledge page that discusses common swadhyaya practices and marks used in various Jain traditions.