Participants in the CAU will receive their baseline depressive symptom severity via text message following randomisation, and they will have unrestricted access to mental health services based on their needs and preferences, such as pharmacological treatments, psychological treatments, and complementary and alternative medicine.

ISI is a 7-item questionnaire that uses a 5-point Likert-type scale to measure the severity of insomnia. The total ISI score is the sum of the scores on the 7 items with a range from 0 to 28. A higher score indicates more severe insomnia symptoms.

The CEQ is a 6-item scale, with the sum of the first 3 items providing a measure of treatment credibility and the sum of the remaining 3 items examining treatment expectancy. Higher scores indicate greater credibility and success expectancy for the treatment.

The TAAS a 10-item self-report questionnaire measuring on a 7-point Likert scale that extends from 1 (disagree strongly) to 7 (agree strongly). Total scores are obtained by summing across all items after negatively worded items (3, 4, 5, 7, 8, and 10) have been reverse-scored. Scores may range from 10 to 70, with higher scores indicating greater acceptability of treatment and greater anticipated ability to adhere to it.

The EQ-5D-5L consists of five items that measure five health dimensions (mobility, self-care, usual activities, pain/discomfort, and anxiety/depression) and a visual analogue scale (EQ-VAS) on global health. Each health dimension has five response levels, i.e., no problems, slight, moderate, severe, and extreme problems. The EQ-5D-5L health states can be converted into a single index 'utility' score using the Hong Kong population EQ-5D-5L value set with a range of 0 (death) to 1 (perfect health) (Wong et al., 2019). The EQ-VAS ranged between 0 (worst imaginable health) and 100 (best imaginable health).

HPLP-II is a 52-item questionnaire scored on a 4-point Likert-type scale ranging from 1 (never) to 4 (routinely). The HPLP-II measures the overall health-promoting lifestyle and 6 subdomains of HPBs, which include health responsibility, physical activity, nutrition, spiritual growth, interpersonal relations, and stress management. The overall health-promoting lifestyle score is the sum of the scores on the 52 items with a range from 52 to 208, whereas the score of each subdomain is the sum of the scores on the specific items (8 or 9 items) within that subdomain with a range from 8-32 or 9-36. A higher score indicates more HPBs.

WSAS is a 5-item self-report scale that aims to evaluate impairment in function in terms of work, home management, social leisure, private leisure, and personal or family relationships (Mundt et al., 2002). The total score is the sum of the scores on the 5 items with a range from 0-40, of which 0-9 will be categorised as low impairment, 10-19 will be categorised as moderate impairment, and 20-40 will be categorised as severe impairment.

The BACE is a 30-item questionnaire designed to assess perceived obstacles, including practical, socioeconomic, attitudinal, and structural barriers, related to accessing mental health services. Participants respond to each item on a 4-point Likert scale from 0 (not a challenge) to 3 (a lot of challenge). The total BACE score is the sum of the 30 items with a range from 0-90. A higher score represents greater perceived barriers to receiving mental health services.

The TiCP is a self-report questionnaire designed to estimate the direct healthcare costs and productivity costs related to mental disorders. The TiCP assesses participants' utilisation of various healthcare services and their frequency in the past three months, which includes all types of visits to primary care (e.g., general practitioner, physiotherapist) and secondary care (e.g., medical specialist, psychologist or psychiatrist, hospital admission), medication costs, and informal care (i.e., by informal caregivers such as relatives, friends, and neighbours) they received. Additionally, information on participants' workplace absenteeism and/or reduction in productivity in paid or unpaid work due to mental health illnesses in the past month will also be collected.